I used a 2x4 (laid with 4" side down) and a square to figure out how much roll out my tires had with X amount of air. I started at 25 psi and went up to 35 psi, in 5 lb increments. There was a bigger difference between 25-30 than 30-35. I forget the exact measurements but there was 3/32" difference with 5 lbs of air change. Also a "race" tire may change differently than a "street" tire.
I bought a 10 ft. cloth seemstress tape to check tire circumferences on both my front and rear tires at different tire pressures
I shoot for 1/4 inch vareince on the rear tires, inflate as needed to get that