I also thougt that it had to be calculated this way, but asked a Tire maker of Dutch origin , because I live in Holland.
They wrote me it was a bit more complicated, and I mailed them back "But is there some formula you use" , and they sent me the formula that the tire-makers in Europe use to calculate pressure for a sertain load, to be found on page 14 of the standards manual 2007 ( that long ago already) of the ETRTO ( European tyre & Rimm Technical organisation).
This formula has a power in it and goes like this
A=B*(C/D) ^1,25 to calculate pressure for a sertain load.
C= D*( A/B) ^0,8 to calculate load-capacity for a sertain pressure.
A= actual pressure ( pressure needed for the actual load)
B= reference-pressure ( pressure at wich the maximum load may be carried up to maximum speed of tire, or if lower 99m/h.
C= Actual load-capacity
D=Maximum load of the tire.
^means to the power, ^2 means square and ^0,5 means root, power 1 means the number itself.
So "your"calculation can be put in the universal formula as ^1.
After I declared this formula holy , I learned myself Excell to make spreadsheets for it, and translated a few to English to go worldwide with it.
Found out , by reacting on several American fora , that it was not that holy, and found an article of J.C. Daws with another suggestion of calculating.
This states a part of the load being carried by the construction of the tire, and the rest is a is your calculation.
But I am still not happy with this , so constructed myself a formula that comes , to my opinion the closest to the ever to be constructed ideal formula.
I will give you the link to a map on my public map of skydrive, where at the bottom "myownformula".
https://skydrive.live.com/?cid=a526e0eee092e6dc#cid=A526E0EEE092E6DC&id=A526E0EEE092E6DC%21601
From there you can navigate my complete public map, like in a forum.
To download a spreadsheet, click on the line of it, but not on the name of it.
Then in the right barr at download and open it , after evt viruscheck, in Excell or compatible to use it. If you press wrong, it will be opened in the browser, but most spreadsheets dont work in it, then go back at the top to skydrive and try again.
But then you still have to take some reserve in the load , but if you take to much, things go bouncing, and gripp gets lost.
I myself introduced the load% for that, and that is the % what the real weigth is of the weigt you calculate the pressure for.
Under 85% L% things begin to bounce, over 100% L% tire damage begins.
Study the link, and if questions pup up , ask me here