I can certainly track my exposure, the question was how to calculate it ahead of time.
No, that isn't what the question was. It is a good question, but not what you originally asked. It may have been what you wanted to know, but it was not what you asked.
To add to what RJP said earlier, I've (for fun) tried using the PADI Nitrox tables to see if I can construct an NDL dive series on 32% or 36% that will hit the oxtox limits. I can sort of get there using 36%, but cannot with 32%. I was doing things like NDL dives to the MOD for 36%, normal surface intervals, then doing it again. So on dive one to 90 ft for 40 mins you pick up 30%; 1h SI lets the next 90 ft dive be for only 21 mins, so you only pick up 15% on that dive. So 3 more 21 min dives to 90 ft, with a 1h SI in between, gives another 45%, for 90% total. So, using 36%, 5 consecutive dives to 90 ft, all to max NDL, and 1h surface intervals, gets me to 90%.
Doing the same strategy with 32%, first dive is to 110 ft for 25 mins, giving 20% O2. 1h SI, second dive can only be to 110 ft for 13 mins, giving 10% O2. 7 more NDL dives like that and I'm at 100%, but I had to do 10 dives totalling 142 minutes BT plus 9h of SIs, for a LONG day!