I like to use childrens sidewalk caulk. Rub an inch wide stripe across the tread, drive in a straight line a few feet till it rubs the caulk. If center of tread rubs clear first, lower pressure. If edges rub first, increase pressure. Couple pounds at a time. You are trying to get the caulk to rub evenly across the tread surface. This works for different size tires, ply ratings, or weight distribution. Front to rear may vary several pounds. With my 37 by 14.50s I ended up at 24 front and 15 rear. Your results will vary from others, depending on ply rating, tire size, rim width, and load distribution. Good luck.