Actually, it's not. Sorry, it isn't, and I doubt very much you are going to go through all of that just to figure out what pressure to use in your tires! Here's a better way: locate the min/max pressures suggested on the tire sidewall. Subtract 10psi from the MAX psi. Badabing badaboom. BTW there should be a really good reason to have different sized tires f/r on a bike.