@stuartv why ppO2 of 1.4?
remember that IMO the biggest benefit of standard gases is mixing convenience *provided you don't do ratio deco, where the ease of that is definitely the biggest advantage* so while you may end up with 28% for your ideal fO2, if you blended it the way we blend standard gases you'd end up 28/13. That would be perfectly fine to choose for that depth if you want 1.4, though I don't.
Two big arguments against standard gases are the fact that they target a bottom ppO2 of 1.2 instead of 1.4, and they put a lot of helium in compared to others by keeping the ppN2 around 2.7.
The question was to figure out what you thought was the "best" ppO2 and ppN2 to target to see what real benefit you thought you were getting.
Why 1.4? Because it gives me a longer bottom time and less deco than 1.2.
I take my cylinders to get them filled. I tell them what mix I want. I come back, analyze them, and take them away. It costs me the same and takes the same effort from me whether I get 28/20 or 21/35 or 18/45 (apart from the difference in cost of the helium itself).
So, 1.2 is DISadvantageous. Ease of blending is irrelevant (to me). I don't do Ratio Deco, so no advantage for me there.
Having a gas that gives me a longer NDL or less deco is advantageous to me.
And, finally, if I'm doing something where a Standard Gas DOES give me some advantage (e.g. blending, or it's available in banked form or whatever), my Best Mix training totally permits me to use that gas and have that advantage. If I were limited to only using Standard Gases, then there would be many times I would have gases that are less optimal than with Best Mix - for no advantage.
This debate seems to be sort of like people only learning to drive an automatic transmission and insisting that that is better because it's easier. Yes, Best Mix means I do one simple calculation to determine the optimal gas for my planned max depth. SG is easier because you just look up the gas on a chart. After that, it's the same (presuming you're planning using a computer). Learn the one extra calculation and you can drive either kind of car and choose which is right based on what you're doing. You can still drive an automatic, if you want to. Learn only the automatic and you are limited, in comparison.
TBH,
@tbone1004, I feel like you aren't a true Standard Gas person. You are simply an "easy blending" person. That means you happen to usually use Standard Gases, but it's a function of logistics/convenience, not that limiting yourself to Standard Gases has an advantage, per se. I have no problem at ALL with using a gas that is a few points off from "optimal" for the sake of convenience. Or even for the sake of money. I have dived Air a number of times when Best Mix would have been 24 or 25%, simply because it's cheap and easy to get an Air fill. And then dived on Air to 150 or 160! OMG! Because my training allows me to do that without throwing off my method for calculating deco.
I only have a problem with the statement that Standard Gases is "more flexible" or that Standard Gases somehow allow a diver to get in more dives than they might if they use Best Mix. That is just bunk.