I'll start with the caveat that you should not be diving deeper than your certification and training allows. With most agencies, Deep Air, Extended range, or Deco Procedures and Advance Nitrox certification carries a limit of 150 ft to 170 ft depending on the agency. So get proper training first and stay within the limits of your training. In my opinion, using a PPO2 greater than 1.4 on the bottom is a bad idea and even going to the 187 feet on air that this would allow is in my opinion a really bad idea as nitrogen narcosis is just too significant at that depth.
That said, the optimum mix from a minimum deco standpoint if you only want to use one deco gas is usually in the 70% to 80% range depending on the profile.
But there's more to consider than just deco time. It's hard to partial pressure mix full bottle of 80% unless you have a boost pump. 70% is more do-able but not that much. My preference for a single deco gas is 50% as it does not give up very much in terms of deco time compared to 70% 80% (and gives up nothing to 100%) as you can get on it at 70 ft rather than 20 or 30 ft. You also don't take as heavy a hit on the CNS clock and it is a lot easier to pp mix as you only need 1101 psi of O2 for a 3000 psi deco bottle. Doing the stops from 70' up on 50% also allows you to use more backgas on the bottom without cutting into a 1/3 reserve.
Advanced Nitrox/Accellerated Deco training is worth the effort and is the way to go as, for example, a dive to 150 ft for 25 minutes using 25% for the entire dive would require a 70 minute run time and 113 cu ft of gas (.6 SAC on the bottom, .4 for deco).
In comparison the same dive to 150 ft for 25 minutes with 25% for bottom gas and then 50% for the deco is possible with a run time of 54 minutes, keeps well over a 1/3 reserve on twin 72's or 80's (only 87 cu ft required) and allows a 1/3 reserve in deco gas as well, even on a 30 cu ft deco bottle.
I get an identical run time using 100% O2 for the deco gas but end up using 14 cu ft more backgas and have to shorten the dive a minute or two to keep at least a 1/3 reserve if using twin 72's.
Using 80% will cut a couple minutes off the deco, but again has almost as large an impact on the backgas available for use on the bottom as you can't use the 80% until you get to the 30ft stop.
Using both 50% and 100% deco gases on this same dive shortens the run time to 48 minutes, leaves you bags of deco gas and leaves you the same additional bottom mix as 50% alone.
Personally, I like this approach as it still allows me to complete the deco relatively quickly if either the 50% or 100% is unavailable (reg malfunction, lost contents, handed off to an OOG and soon to be ex-buddy, etc). One important thing to consider is that you always want a redundant supply of gas adequate to complete your deco in the event you lose one of your primary deco gases. If you are using only one deco gas, you need to have an adequate reserve of backgas to complete the much longer deco required. I don't believe in relying on my buddy's gas supply for my redundancy as if the same event happens to us both, we are then both screwed.
In moderate deco with run times under an hour, the 1/3 reserve in backgas should allow you enough gas to complete the deco in the event you lose one or both of your deco gases. If you are doing extended deco where you do not have enough reserve backgas to complete the longer deco required if you lose your deco gas, it's usually a powerful argument for going with 2 deco mixes or at least 2 separate deco bottles to ensure you can fully decompress in the event you lose one of them.
I put the primary dive plan on a slate and then put the contingency deco plans on subsequent pages as well to allow quick and easy reference to whatever deco plan fits with the operable/available gas(ses).
