pick the range that you have most common, for most divers it's typically 31-33%, set the computer for 31, and call it a day. Unless you are diving those gases where the ppO2 is 1.6, the EAD is not going to be that different for NDL's.
You said +2% and mentioned 31 and 32. So from 29-34
MOD is the most important thing to abide by here, and at 1.4, the MODs are below. You obviously have to be very careful when venturing below 100ft on Nitrox for what gas mix you're getting due to oxtox risk.
EAN29-126
EAN30-121
EAN31-116
EAN32-111
EAN33-107
EAN34-103
For NDL purposes, you need to be cognizant of EAD since that is essentially what the computer is tracking and is the benefit you get in NDL's while on Nitrox. Let's assume a 110ft/33m dive because it makes the math easy for me. The EADs are below. As you can see we have less than a 10ft range across the entirety of the mixes which is not going to equate to any real change in the NDL's. Heck, even on the NAUI dive tables, the difference in that dive is only going to be 3 minutes of NDL time if you are following it strictly. The difference between actual depth and EAD is obviously even less as you get shallower.
EAN29-94
EAN30-93
EAN31-91
EAN32-89
EAN33-87
EAN34-86
For what to really do? If you are regularly getting mixes in the 30-33% range, and are not at risk of violating the MOD's of any of the mixes, just set the computer to EAN30 and be done with it. No need to change gases in the computer for 1-2% in your mix. On 50/70 which is what I dive, Decoplanner says 14mins at 108ft on EAN29 is NDL, 16mins on EAN32, and I get 17mins on EAN34. Hardly worth the effort in my opinion to change gases for a minute here of there. If you are running more aggressive GF settings, then it will spread out a bit more, but not by more than a few minutes. To me that isn't worth the hassle of changing gases, and running a bit more conservative never hurt anyone.