Found your question when searching for tire/tyre-pressure , wich I do every day. registered to give an answer.
Live and was born in Holland ( Europe) and once got hold of the officially used formula for calculating pressure for a sertain load, and went running with it.
Got much wiser in time about it and tires by the internet, but also by nagging tiremakers and others specialists on internet about it.
Even nowadays dare to call myself tire-pressure specialist ( but sertainly no tire-specialist).
Dont own a Ford Transit so dont think its usefull to introduce myself in a seperate topic.
Tire pressure advice is all about to give the tire for the speed used , a not to high temperature at any spot of the rubber so it hardens. Then if this damage is done , the tire crackes at those spots when the rubber bends, because the tire deflects.
Mind that every segment of an average tire deflects and flexes back about 10 times a second when driving 50m/80km/h.
To give the tire a deflection in cold condition that wont give to much heatproduction for the speed , there has to be a sertain pressure for the weight resting on it.
In time I constructed my own universal formula , of wich the European officiall formula can be made by putting the right power (P) in it, and the ( babtised by me) construction load,( Lc). but also an alternative one I found described by an American IR J.C.Daws, in his article where he compares it with the formula used in Europe and America.
To make it short I will copy text I saved , so I dont have to write it all over again.
Tirepressure advice is all about load on tire and speed ( and sometimes about alighnment - camber angle).
So if you can give details of car and tires , I can calculate an advice pressure with some reserve for things like, pressure-loss in time, unequall loading R/L, incidental extra load, misreadings of pressure scales,and misyudging of weight, etc.
This is from tires next and can be read from sidewall:
Maximum load or loadindex.
Kind of tire to determine the AT-pressure/pressure needed for the maximum load up to maximum speed of tire, or if lower 160km/99m/h/reference-pressure, wich is not the maximum pressure of tire.
Maximum speed of tire, most given as letter ( Q=160km/99m/h,N=140km/86m/h fi)
If you have offroad or tires looking like that , with large profile blocs that cover a part of sidewall, also mention, they are allowed lesser deflection then a normal road tire, then the tire maker used to determine the maximum load (to my conclusion the case for the Bridgestone tires on Ford Explorer in the Ford/Firestone affaire).
If you cant find all of it give sises of tire and Loadkind, then I will google for it.
From car next and mostly can be found on same plate as the original pressure advices:
GAWR and GVWR ( Gross Axle/Vehicle Weight Rating)
But best would be to determine the real weights in your use on seperate tires or estimate it as acurate as possible, by weighing per wheel(pair) or axle.
Maximum speed , you dont go over for even a minute in your use, eventually different for different situations, for instance when towing or fully loaded.This apart from trafic regulations, if you drive faster then allowed give that speed. Nature punnisches with tire-failure, police only with a penalty.
Give all that and I will calculate and give a picture of one of my filled in spreadsheets in my answer.
If other then original tires, indead as is already answered other advice is needed, a stiffer tire ( fi C-load instead of P-tire) needs a higher pressure for the same load, or the other way around, has lower loadcapacity for the same pressure.
Greatings from a Dutch pigheaded self-declared tire-pressure-specialist.