3th possibility is to let me calculate it for you with my Motorhome-tire-pressure-calculator.
For that I need from vehicle.
Seperate weighed wheel loads in the condition you load it, but Axle weighing can do.
If you dont have one of those , we will have to do with the GAWR's( gross axle weight ratings), asuming that you are not allowed to go over them, but for rear it happens, and I take a large reserve to cover that.
Then also give GVWR ( gross vehicle weight rating)to make it complete.
From tires.
Maximum load or loadindex ( when different kind specifie)
Loadkind so for instance C-load/6PR or E-load/10PR or mayby yust P-tires or XL/reinforced/Extraload, to detemine the pressure needed for the maximum load, also called maxloadpressure or reference-pressure, wich is not the maximum cold pressure of the tire.
Then also give sises and speedcode of tires ( letter between J and Q or S,T,H,V,W,Y)
And camber angle of rear tires mostly.
Also yudge if the tires are for off-road use or have the looks of that, with large profile blocks covering part of the sidewall.
Then I can calculate the lowest save pressure and first add a reserve to the loads that gives savety margin but still comfort and gripp is acceptable.
About the TPMS , it can be reset to an other pressure , sometimes even the reserves can be set too. Every system though different manual , so you have to search for that.