Pressure in spare

Hello All!

Does anyone know how much air to put in a spare tire while it's still on the spare tire rack?

The tire says max 50 psi, but that would be with the weight of the trailer on it.  What about with no load?

I've been filling it to about 40 psi thinking that should I ever have to put it on the camper,  I can either add a little or let a little out before driving.  I'd like to get a more accurate calculation to help narrow it down a little.
Thanks alot.

Gary