Thanks,
OK, that's not a new one for me - however, it is one I debunked quite a while ago. Here are the flaws:
1) The max pressure on the tire is NOT related to max load - at least for P type tires.
US regulations say that the max load and the max pressure must be imprinted on the sidewall of any tire that is to be used on the street. Usually it is expressed this way: Max Load XXXX at YY pressure. Notice the relationship. This is the way pretty much the way every tire is done except for P type tires.
For P type tires, it is usually expressed this way: Max Load XXXX, Max pressure YY. Notice that no relationship is expressed - and there's a reason for that. The pressure where the load maxes out for Standard Load P type tires is 35 psi (or if using metric units, 36.3 psi), but it is permissible to use higher pressures for certain conditions. The values specified by The Tire and Rim Association are 44 and 51 psi. I show where those values come from here (about 1/3 the way down):
http://barrystiretech.com/loadtables.html
2) The load curve is NOT proportional to the inflation pressure, nor is it linear. So the formula won't match what the tire manufacturers (through The Tire and Rim Association) publish.
3) Nowadays, it is common practice for the GAWR's to be 85% of the load carrying capacity of the tire at the placard pressure. This is a change that result from the Ford/Firestone situation back in 2001.
The net effect is that you won't get what the vehicle manufacturers specify. The fear is that you will get a value lower than what the vehicle manufacturers specify and that could be dangerous.
Plus, the spring rate of a tire is relatively proportional to its inflation pressure, so the vehicle will handle differently if a different pressure is used.