My reading has brought me to believe that you use the side wall as a guide. There's 2 numbers on the sidewall you want to look at. The psi and that is always at a specified weight. So if I pull some numbers out of the air, your tire says it's rated for 50 psi @ 2000lbs you have to look at your load and say... ok my truck is probably only putting 1000lbs on that tire - which is half that rated weight. So I'll start at 25psi as a baseline and see how it wears. Marking like Bent said earlier will fine tune it from there if you want to go further.
Temp does matter - check your psi cold.