That is a great question!
I start with the vehicles specs, never the tires.
My reasoning is that the max psi# by the tire manufacturers does not take into consideration that all vehicles do not have the exact same weight loads and are not always the ideal 50/50 weight ratio.
I can then increase the rear a bit for extra passengers, heavy cargo or when pulling a trailer (if I ever find a used OEM hitch)
I'm sure that after my post there will be a really smart engineer that will give you a much more complicated and more precise explanation!