This is referenced in the Note to Instructor in the Deep Diver Instructor Guide in section 2 part 2, that the instructor should review air consumption calculation with the students, and work through a number of example dive plans so they can apply their gas consumption rate to different dives.
The student material in the "Deep Diving Techniques - Air Supply Control" section has an entire section on how to calculate gas consumption rate, and it is followed by another section that specifically references the difference in appropriate gas reserve from the "500 psi back on the boat" idea. It suggests several additional variables that will require more reserve gas, and suggests using rule of thirds for gas management on deeper dives.
Hope this answers your question.
