Heine several years ago promoted a 15 percent tire drop theory for optimum tire pressure. I usually run higher but for touring my rear pressure was probably pretty close to that recommendation when I toured with 40mm or narrower.
https://www.adventurecycling.org/default/assets/resources/200903_PSIRX_Heine.pdfFor touring at teh higher weights, I had to get out the straight edge and a scale, extend the lines further to the right for the higher load on the graph. My 700c touring bike has 37mm tires, I sometimes tour on 40mm wide 26 inch tires and for the purpose of the graph you can sort of guestimate where the 40 line would be by looking at the 32 and 37 lines.
I have also toured on 50 and 57mm tires, I do not try to figure out what the 15 percent drop should be, I know I am running higher than that.
I agree with MartinF, I usually run front about 80 to 85 percent of the rear, sometimes a bit lower. But the 15 percent tire drop recommendation I thought was absurdly low for the front, so I never ran front pressure that low.
Exception, I was in Texas Big Bend area a couple months ago, they have a really rough chip seal pavement that is rougher than some gravel I have ridden. I was not carrying much gear on the bike, a handlebar bag and a saddle back with some extra water. No camping gear on the bike. My first day my hands were taking a beating and my GPS on my handlebar started working erratically from the vibration. I was running about 80 psi in the rear on 40mm wide Schwalbe Marathon tires, running between 55 and 60 psi in the front. Day two, I dropped my front pressure to between 40 and 45 psi, but kept the rear the same, that day my hands were much more comfortable and my GPS got back to normal. The rest of the week I stayed at those pressures.
