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You can fill in spreadsheet I made , in wich I use the formula that the tire-makers in Europe also use for decades.

https://skydrive.live.com/?cid=a526e0eee092e6dc#cid=A526E0EEE092E6DC&id=A526E0EEE092E6DC%21348

in this map the spreadsheet.click on the I in a circle at the end of the line, then Download in the right barr and use it in Excell or compatible.

The pressure mentioned in that table is what is called in the formula the referencepressure.  At this pressure the maximum load can be carried up tot the maximum speed of the tire, or if lower 160km/99m/h.

This is not the maximum pressure as it is written on the sidewall of Standard load and Extra load ( XL, reinforced the same) tyres.

So if you have lower loads then the maximum . you can calculate with the formula the needed pressure for that. But always take reserve of say 10% for misreadings of pressure and load, pressureloss in time, ocacional extra load, loadshifting and unequal loading R/L.  If you go to far with the reserve ( add more then 18%) thing begin to bounce.  Trick is to keep the real weight on the seperate wheels between 85 and 100% of the weight , you calculate the pressure for. The more accurate you know weight and pressure, the more you can hang to the 100% for more gripp and comfort, but savety first, so if you dont know almost exact, use about 10 to 15% reserve, or make the Loadpercentage, as I babtised the % above on 90%. in case of 4 point weighing keep the lowest weight per axle on 85% and check if the highest stays under 100%, wich it will then mostly.

This adding of 10% is what in America is done for Standardload tires on MPV.But there it was done for another reason.  It was to compensate the wrong calculation they used before 2006 for Standardload and Extraload tires, is my conclusion of the "Ford/Firestone-affaire". That is why you  must not trust old american tables of pressure for standard load, wich are still spread on the internet.