If you are breathing plain air, the O2 content is 21% or .21.
At two atmospheres (33 feet), you double the pressure to .42
At three atmospheres (66 feet), you add another .21 to it and get .63
This is your PO2 at that depth on plain air.
You can see if you are using 32% nitrox, you would have to use .32 instead of .21 in your calculations. You would reach the safe maximum PO2 of 1.40 at a depth a lot sooner than if you were on air.
Going to a 36% mix would put you at a PO2 of .72 at only 33 feet. (.36x2) Hope that math is right, I have had a glass of wine.
Anyway, as your depth increases, your mix (% of O2) becomes more critical in your calculations. 1.6 is absolute max and 1.4 is acceptable.