the simple rule of thumb I use is to set your psi so that each tire's contact patch is about one square inch.
this is easy math:
bike+rider = 200 lbs
front/rear = 55/45
.55x200 = 110 rear psi
.45x200 = 90 front psi
If your math works out to a rear psi higher than 110 or so, it means you need bigger tires.