but, with that said, let me offer a few suggestions..
First, the real issue is comfort and familiarity. Familiarity with your air consumption, for example. Gas goes faster the deeper you go. The more you burn normally, the shorter you can stay before you go "oh oh, I'm out of air!" Since that is a REAL serious problem at 100', and is an almost non-event at 30' (most people can make an ESA from 30' without trouble) this is a very real consideration. At minimum you need to know how much gas you need to make a normal ascent from your planned depth, where that pressure is on your SPG, and then
at least double that for your turn pressure, because if you have trouble either (1) your gas consumption will be up significantly, (2) you will be sharing your remaining gas with your buddy who has had a problem, or (3) both![/b][/i]
A
truly ugly scenario is a double-failure; that's how people die. An example is your buddy goes OOA, you hand him gas, you start your ascent, and then as a consequence of improper gas management you
both go OOA on the ascent! The "pucker factor" of the second OOA once you're already stressed can easily lead to either you, your buddy, or both of you either being seriously hurt or dead. Your dive planning should, IMHO, be focused on preventing
double failures from occuring - equipment breaks and things go wrong, but the first problem is rarely the fatal one. The second, third or fourth one in a row is usually the one that kills you.
Buoyancy control is a far bigger deal on deep dives. If you make a rocket-ship ascent from 100', you are very likely to end up bent or embolized. If the same thing happens from 30', assuming you don't hold your breath there is a good chance nothing bad will happen to you at all.
I was diving to 100' within a few months of being certified. I do not seem to notice narcosis at all at 100', but at 110', I do begin to.
In general my rule is "change only one thing at a time", and if diving with new gear or otherwise under changed circumstances, make the first dive with LESS task loading than your previous dives. As an example I do a lot of spearfishing. When I started diving dry, my first couple of dives were made WITHOUT a speargun - just to look around. Only after I was comfortable with the suit and how it changed things underwater did I take the gun once again, and again go on the hunt.
Finally, if you're going to dive "deep" (beyond the 60' boundary) give careful consider to both your gear and your buddy. If you do not have an ABSOLUTELY reliable buddy you NEED a fully redundant source of gas - particularly below 100'. I did without this for a while, even on deep dives, but it started to bother me the more I considered whether I'd be able to make a 100' CESA, and if I did manage it, how badly I might get bent doing it. The options that make sense are a sizeable pony bottle, slung like a stage (NOT a "spare air" or a small pony!) or a set of manifolded doubles, with the additional weight and bulk they come with. As a consequence I now carry a 19 cf pony, slung like a stage bottle, on triple-digit dives or any dive where I do not have a reasonable expectation that a buddy will be within immediate assistance distance - which, when spearing, is most of the time!
I thought that the slung bottle would be in the way (it IS on the surface!) but it nearly DISAPPEARS underwater; the back swings up under your upper arm, and for all intents and purposes its not there underwater... but if you need it, the 19 cubes of gas will allow a normal ascent from 130', at 30fpm, with a 3-5 minute safety stop, along with a decent safety margin to allow attempting to resolve the original problem and/or a significantly-elevated SAC due to your normal "the water is brown" response. I fill the pony with air rather than Nitrox, because (1) there is NO MOD issue to deal with if I decide to breathe it, (2) I'm headed up if I need it anyway, so the effect on NDLs should be minimal to zero, as I'm not sticking around to accumulate more nitrogen once its been "drawn", (3) I can safely hand it off to another diver who has an OOA or other issue, even if they're only air-certified, without concern as to whether THEY will respect a MOD on the gas or not, and (4) it is trivially easy to top it off without re-mixing from my compressor on every fill cycle, which encourages me to test its use and deployment on a regular basis - an emergency device you never try may turn out not to work when you really need it!