1) You don't really understand your RDP tables at all.
They were made with a computer, and you don't understand how that was done any better than you know how your computer works. In fact, one extremely popular computer is exactly the same algorithm as that used to produce the RDP. Were you to dive a square profile using assumed ascent/descent speeds, they would correlate to within seconds on exposure recommendations. Actually, that was true several years ago, but the company has adjusted the algorithm several times since then making it more conservative than the tables.
Case in point: you don't know they are the same to begin with, ergo you don't "understand" the tables at all. What you know and value, is how to work them.
2) On the surface you're guessing at your depth; while diving OTOH, your computer knows your ambient pressure exposure to an accuracy better than one part in three thousand per foot (pascals).
Which do you think is better: Certain knowledge, obtained, measured and scientifically, unemotionally applied, or a SWAG by an inexperienced diver?
By proxy, your computer is offering you the advice of dozens, no hundreds of experts in diving physiology (using, analyzing and testing billions of saltwater-foot-minutes) to interpret your exact diving exposure limits.
You think you know better. You don't, and that's a fact.
3) If by "I Ascend. period"(by your countdown timer) you mean the dive is over , you are making your dive more dangerous, not less dangerous.
Following the deep leg of the dive, the more time you spend (within reasonable limits) at shallower depths (where no-stop time is not running down), the better. It gives your body time to eliminate the inert gas taken on at high pressure, using a reduced gradient. This is a much gentler way to remove the higher pressure inert gas from your system. You can take air out of your tire by bleeding the air valve (multi-level profile), or you can stab the sidewall with a Bowie knife (leave the water). Your choice.
Your computer, no matter what model, will see to it that you don't push the NDLs, but you have to pay attention. It will tell you far more about your diving than any stupid, sessile table ever will. If you pay attention, you can greatly reduce your risk of DCS with sensible profile management.
Uwatec/Suunto/Pelagic/whatever computers will all recommend the same time limits to within a surprisingly small variance, because the limits and application of the fundamental model are that well known. While everyone in the world will jump on the decompression-models-are-theoretical bandwagon, they[models] have proven themselves millions, perhaps even billions of times to be reliable simulations of human physiological no-decompression limits in a recreational setting and well beyond.
Nothing can protect you from the statistical outliers. Bends happen within no-deco limits because random physical events coincide with random physiological conditions or stresses and that's that. You can't do anything about it anymore than you can predict when an exhaust bubble will split on it's way to the surface or how many times...go ahead and watch some. Thinking that superstitious interpretations will protect you from "undeserved" hits is patently wrong and unsupported by any study.
One thing can protect you from "undeserved hits": don't dive. Unless you have a physical deformity or dive stupidly, the chances of an undeserved hit are as near zero as you can imagine and not be zero.
5) You are wasting your time and money.
Why are you bothering with Nitrox? It is not an economic choice unless you are running out of bottom time before you run low on gas; this is not possible given your preference so you are making your diving more complicated and hazardous for no reason. It doesn't make any sense. The cardinal rule of quality is that removing a decision or step from a process means that you can't make a mistake and vice-versa. Never take unnecessary steps. You are adding all this complication and expense for nothing.
Sorry for the long reply, but these kinds of posts show up far too often and people with less experience than you read them and get the idea that its "OK", unless someone takes it apart and explains the problems. It's nothing personal, it just needs saying.
I'll respectfully disagree with your understanding of what I know and don't know. It's quite clear from looking at a table how they work and the general progression of what to expect. I don't give a damn if a computer was used to generate the tables or not. The tables are a "quick look up" which makes it easy to see more than one data point simultaneously. Computers, or dive computers, only provide one data point at a time so that's harder (impossible) to do.
Despite your incorrect assertion, I DO know that computers are based on the tables, but I can't see how the computer is arriving at its calculations. There's a big difference between those two things. If I had the algorithm in front of me, then I would know both things, but I don't have the algorithm in front of me (and I suspect computer manufacturer's don't share that information) so I CAN'T know exactly how it works.
As to guessing at my depth, when I dive a square profile (most of my dives so far, to be honest) there is no guess-work. Tables are probably more than sufficient but I use a computer during the dive anyway. Yes, if I'm doing a multi-level dive my "average" depth guess is perhaps a guess, but my dive plan isn't a guess. I know what depths I will hit and for the most part have a good idea how long I will be at those depths. As such my "guess" is more like an educated estimate than a guess.
As for the fact of me not knowing better, I would agree. I completely disagree with your assertion that I somehow think I know better than the designers and far more experienced "experts". If you had read my posts carefully, you would have seen that this "check and balance" that I perform is to educate myself, not because I think I'm being more conservative. I don't know so I choose to compare two different sources rather than sole-sourcing my life to any single piece of gear.
Please explain how ascending from any recreational depth within NDLs (arguably well within NDLs) is somehow "more dangerous". "Ascend now" does not mean skip safety/deco stops. Your assumption that it does is quite telling. You've also made the assumption that I stayed at the deeper depths my whole dive, which so far has never been the case on my multi-level dives. I will grant that my original post wasn't exactly clear in what I meant though, so take this as further clarification. When I said "I ascend, period" I mean, I ascend at an appropriate rate, making appropriate stops.
As I've said in other posts, this isn't about me distrusting the computer. This is about me knowing/learning what is a reasonable thing to expect without having to trust the computer implicitly. Education is an amazing thing, and this habit of mine has been about becoming educated. I trust my computer(s) just fine. That said, computers do screw up, batteries go dead, floods happen. In the event that one of those things happens and I'm at 70 feet and it somehow reads 40 minutes of NDL when I have 1000 PSI left on an AL80 and have been diving for 15 minutes, I want to know that something is awry.
I will repeat, this isn't about "preventing the bends"... it's about understanding how it all fits together and knowing when something isn't right.
As for Nitrox, I chose to take the course because I have an upcoming trip where I will be making multiple dives per day for roughly 7 days straight. Whether it's true or not, some people believe they are less fatigued when diving this way using nitrox. The science, at least on the surface, seems to make sense with this belief. I figure either way it won't hurt to use the nitrox and the education is good to have even if I don't use it. If an extra $5 per tank is somehow going to break my wallet on a week-long trip then I probably wouldn't be going on the trip. Economics has no part in my comfort.
Thanks for you thoughtful response. I disagree with some of your assumptions but the effort you took to make your points for us newbs is appreciated.
