Several years ago someone suggested a 15 percent tire drop theory:
https://www.adventurecycling.org/def...SIRX_Heine.pdf
That theory fails on really wide tires like 2 inches or wider, but in general terms for touring I have used pressures in that range for 35 or 37mm wide tires for the rear tire.
Note that it is weight on each tire, not total weight. I do not follow that theory for the front tire, I think that gives too low a pressure. Instead I usually run a front tire at about 70 to 75 percent of the pressure that I have in back.
That said, on a really rough pavement if I have a lot of vibration on my hands and handlebars, I will run lower pressure on the front to smooth that out.
If I have a tire that has a very stiff sidewall or tread, I might increase the pressure a bit to make the tire deform less, because deforming a very stiff tire is lost energy, some tires are slower than others.