Chalk method for tire pressure?

[chevmn]

Some would cringe at the pressure I used to run on 10-15 tires on 8 inch rims on a 67 C10 on the rear (empty). 4 ply load range B tires. Unloaded I routinely used to run around 10 PSI. I basically looked at sidewall flex, and inflated to just where there was some bulge in the sidewalls. Keep in mind, I rarely was running any interstate, and the speed limit was 55 at the time. These were bias ply tires (no one ran radials yet in the 70s.