I learned a long time ago that air pressure in tires depends on a lot of factors (chassis or car in question, weight, weight of cargo driver / tires etc). Honestly the way I've done it for years is to get a load rating table from the manufacturer who makes the tires you are using - there are also psi calculators that help.
Look up the original load range (door tag which lists factory tire type and recommended pressure). Then get the load range from your current tires and go from there.
You can also use this
Easy to use tire pressure calculator. Use our tire pressure calculator to calculate the new tire pressure needed when changing tire sizes.
tiresize.com
I will tell you that most people use way too much psi in their tires. Funny story about tire pressure. I bought a 1st gen Miata for $150. Yes you read that right. $150. It was the price offered by the scrap yard a friend's neighbor was taking it to. Supposedly it had a blown head gasket. Nope - it had a busted radiator hose and a few other small things wrong with it. it. It DID however have brand new tires just put on at a local tire shop. First test ride I just KNEW something was messed up in the suspension of that car. It was ALL over the road.
Turns out that Miata called for like 26 or something psi from the factory. New tires were at 40psi. Lowered the pressure. Car drive / handled great. Sold it the next week for $1500.
Hope this helps.
