If I wanted to (finally) calculate the real pressures at maximum van weight capacity would I use this formula for each axle ??

Max load per tyre x 2 (stamped on tyre) = max load on that axle

then

Max axle tyre load/max rear permissable axle load = ratio

then

max psi of tyre/ratio = psi to inflate to

So using the details from the westfalia spec sheet for a high top joker

Front axle permissable load = 1100kg

Rear Axle permsisable load = 1300kg

and my Tyre max weight 750kg @50psi

front axle

750kg x 2 = 1500kg max tyre load

1500/1100 = 1.36

50psi / 1.36 = 36psi

rear axle

750kg x2 = 1500kg max tyre load

1500/1300= 1.15

50psi/1.15 = 43psi

So as I am never going to be running at max weights I should be fine using a small rounding down factor to say 34 fronts and 40 rears?