I take the weight rating of the tire at it's max pressure and divide the weight by the psi. Then multiply that by the weight on each tire. That gets you close and can adjust from there
For my truck. Front is about 4500 lbs. That's 2250 lbs on each tire. My tires are rated at 3750 lbs at 80 psi. 3750/80 = 47 lbs per psi. 2250 lbs / 47 = 48 psi in each tire.
Rear is something like 3500 lbs. Since mine is a DRW that would be 875 lbs per tire. That works out to about 19 psi per tire
I think in the end I settled on 45 psi in the front and 25 in the rear.
Adjust for your tires rating, vehicle weight, tire wear and handling, etc. I think for most SWD vehicles with lower rated tires will come out close to what Tom uses