I registered to this forum to give , what I think , usefull information about tires.
End of 2007 I got hold of the formula , the European tyre-makers use to determine the advice-pressures for cars, and went running with it.
Learned myself Excell to make spreadsheets for it, and translated a few from Dutch to English to go worldwide with it.
Even last year made new spreadsheets that are multi-lingual, so you can translate from English English to American English if you want.
By reacting on several fora, came to the conclusion that this European formula was adequate for normal and XL tires , but gave to low pressures, or to high load-capacity for C-load/6ply and up . Your tires here C-load with reference-pressure of 50psi wich is not the maximum pressure of the tire, tire-makers allow 10psi higher for better handling and higher speed then given on sidewall. Normal car tires and XL/reinforced/Extraload give only maximum pressure on the sidewall, reference-pressure to calculate with is 35psi and 41 psi ( XL) in America.
To make a long story even longer, look around on my public map of skydrive , that belongs to my hotmail-adress, with the same username as here.
There are a lot of English maps between the Dutch.
One for Caravan/trailers in wich I also give the tonge-weight percentage.
It calculates a pretty high pressure , for reserve for loadshifting, pressure-loss in time, miss-readings of scales, misjudging of weight etc.
I based the reserve of 10% adding to the GVWR, going from the Dutch and probably European situation. Most caravans, as whe call the houses tot tow behind a normal car, have a towbarr weight of 50 to 90 kg/ 110 to200 lbs and so about only 5% tonge-weight. For your 10 to 15% the warnings have to be different I think, but you could fill in the GAWR instead of the GVWR for same reserves. Then the calculation of the tonge-weight goes wrong, so better lower the reserve in the info sheet to say 7% to get better division.
the weightdifferences at wich tire-damage begins or bumping still are what they should be.
But also I am working on a CamperRV calculator, all results placed in that map. Pretty reddy now, only needs a lott of explanation in the info sheet.
So see what you can use for the topic here.
Greatings from Holland, and If you cant work it out, give me the details, and I will do it for you as an Example.