Disclaimer: This is MY view, and it may, or may not, be shared by the agencies I've been trained by. It may also be considered inappropriate by the mods. However I don't see any issue with my thinking and therefore feel free to share it. Do not go on "trust dives", doing something "because some guy on internet said so".
I round to whatever I feel like (ie lower if I happen to stop clicking the button too soon, or higher if I'm a bit slow to stop. Heck I generally even leave it on air when diving shallower than 25-30m and less than 3 a day)
- Deco is not rocket science.
- Your analyser is most likely off by a value that can be anywhere from 0 to 1 or 2% (because when calibrating we ASSUME the "voltage to %" curve is a straight line, which is not the case).
- Ox tox is not rocket science.
So in other words: Do as you want, you won't die because of any of those choices.
An example to explain why I think like this:
For deco: let's say 30 minutes at 33meters (yes this is a completely random profile), using N32 and an algorithm that's unlikely to be the best (gf 30/70 on buhlmann), I get on my simulator a total run time (ie those 30 minutes at the bottom + deco + ascent time, yes I mention deco in the basic forum, and those values seem long to me, but heck it's just an example) of 48 minutes. N31 gives 50 minutes, N30 gives 51 minutes. It is up to you to then decide if you think those 2 minutes will get you bent or not. I personally believe they wont kill me, simply because I use the plans as an enveloppe, in the same way you tend to not dive your computer exactly to the NDL, I do not dive my plans exactly to the limit.
For MOD: Assuming a maximum pO2 of 1.4. N32 gives us a mod of 10*(1.4/0.32 -1) = 33.8m, N31 gives a mod of 10*(1.4/0.31 - 1) = 35.2m, N30 gives us a mod of 10*(1.4/0.3 -1) = 36.7m.
And the pO2s at the maximum depth, ie what happens if I took a N32 to 36.6m. 0.32*(36.6/10 +1) = 1.49b, N31 gives us 0.31*(36/10 + 1) = 1.44b.
To me, these differences are small enough to not worry about it.
Again, this is only my view.
I just feel like there's too much worrying about minor things in diving, when major issues are overlooked (are you able to give air ANYTIME to your buddy without him having to come and grab you? are you able to deal with a stuck inflator in whatever position at any time? etc.)