I took the PADI Computer Nitrox class, and don't remember doing any calculations for the test. But after going back and learning what PADI didn't teach me, the things I wish PADI taught be better would be the equations for MOD, and the NOAA O2 exposure tables.
As Tursiops earlier, the PADI nitrox course I took years ago required all sorts of calculations. It was pretty tough. Nearly none of that is in the course today. I will address your two points and add more.
1. If you are using a computer, your computer will give you the MOD. Not only that, if you are diving at altitude, computers will adjust the MOD for altitude. (More on that later.)
2. The PADI course used to have long term O2 exposure tables as part of the course. The only question I missed on the 50 question test involved using that table. One thing I learned from that class was that to get into trouble on that table, you had to be doing a really, really serious set of dives. But that is not all. According to an email I go from PADI earlier this year, research after that indicated that if you are diving at the 1.4 standard, you can pretty much dive all day without a problem. That table was thus teaching something that was simply not necessary. Not only that, I learned not long ago that the NOAA O2 exposure tables is pretty much a wild guess--there was almost no science behind it when it was created.
3. Several years after getting that complex nitrox certification, I got certified as a DM, and I had to learn all the gas laws for that. I worked hard, and I got to know them well. I then did a fun dive trip with several people associated with the dive shop for which I was a DM, and we were diving nitrox at altitude. When we were talking about MODs, it suddenly occurred to me that we were diving at altitude--did we use the sea level MOD, or an adjusted MOD? We decided it had to be adjusted, but this had never occurred to any of the dive professionals in our group. We decided that we had to use the altitude adjustment table used for dive planning, meaning the 85 foot dive we were planning at the altitude needed to be planned as a 108 foot dive. We could therefore not dive with anything richer than 32%.
So that is what a group of highly trained professionals, including two nitrox instructors, decided,
and we were dead wrong. The MOD for 32% at that altitude is 120 feet. We could have dived 40% safely there. Our calculations took us in the wrong direction--the MOD goes
up as you gain altitude, not
down. With all of our training, we did not know how to apply those gas laws we had all learned so thoroughly. In other words,
people who rely on remembering and using that training are all too likely to make a mistake, and they are much better off looking things up.
I dive at that altitude regularly these days, going to depths like 280 feet when I do. Knowing the MOD is important to me. I therefore created a table for that altitude, and I do what every other diver does--I consult the table when planning a dive. During the dive itself, my computer tells me constantly what my partial pressure is during the dive, and it is adjusted for altitude.