Much 99% of OW courses, I was taught the surface w/ 500psi rule of thumb....but I was also taught...."in order to do that, you have to compensste, so you should start your ascent around 700 depending on your depth and safety stop"
This was fine with me and my buddy, but when we were somewhere we weren't familiar with, we decided that 1000 was a good number to use, and followed that rule.
Within the last year-ish I had begun to think...how much gas do I really need for that OOA ow-diver who somehow screwed it up thanks to my DM training. So during my DM class, I started working with SAC rates, and converting pressures/volumes to what I called "Fail-safe psi"...I never really shared it with anyone for fear of looking like a baffon. Recently I took the DIR-F class and I found out "fail-safe" was damn near the exact same thing as rock bottom. The big difference was adding in a deep stop and doing some conservative conversions to make the math easier....whimps....
At Any rate, nearly all the diving I do is in a quarry at less than 100', so rock bottom or fail-safe is what I determine. I figure how much gas/air I'm going to need for myself and my buddy to do a safe controlled ascent....ususally it ends up being around 700psi depending on depth.