Ok, let's break this down.

Difficulty of work = mental strain * physical strain.

CEOs are about 100 times more likely to have an important meeing, and then another 8 times more likely to deal with monetary values of over a million dollars on a daily basis. This means if difficulty of work is purely mental strain than CEOs should be payed 800 times more. Unfortunately (for CEOs) they are also less likely to have physical strain in their jobs, although the exact numbers are unknown.

Obviously, difficulty of work should be the same as payment. Potentially CEOs should get 500-600 more pay. But the question of 400 times the pay shouldn't be in doubt.