VE7DAC
Registered
Don't think of it in terms of just PO2, think of it in terms of P(Narcotic Gasses), or PO2+N2. From a narcosis standpoint, it doesn't matter what the proportions of O2 and N2 are. They're equally narcotic. Breathing O2 at 80', in theory, should have the same narcotic potential as air (it obviously has other, separate issues).
If I want to keep my END at 100 or less, the maximum PO2+N2 I can have is 4.0. If I'm shallower than 100, problem solved. Just watch the PO2. If I'm deeper, just subtract that 4.0 from ambient pressure and convert the remaining balance to FHe.
That's a handy rule of thumb. And it makes sense that oxygen can contribute to narcosis, my initial confusion was about how much it contributes. Since it's not considerably narcotic on its own at 1.6 PPO2, I didn't consider that it would compound the narcosis from the nitrogen.