Hmmm that makes sense. By looking at the partial pressure one could adjust the total pressure and the gas mix to obtain the desire ranges.
Well, not so much the "total pressure" as I understand it, but definitely the mix (fO2). ppO2 at 1.2-1.4 generally defines MOD. 32% nitrox has 0.32 ppO2 at the surface (1ATA), and 1.4 ppO2 at 110ft (just over 4ATA).
Similarly, a mix with 10% O2 is not breathable at the surface, but should become so before you hit 33ft or 2ATA (at which point the ppO2 becomes 0.20, or roughly the same as the ppO2 would be at the surface).