There is no exact rule for seat height. After following a certain set of general guidelines, you'll have to just raise or lower you set in cm or smaller increments until you find that sweet spot. The more you ride the more nit picky you will get about seat height. A good starting point for me is to have my seat high enough that I can just reach the pedal with my heal in the 6 o'clock position...Ride like that for a while and adjust accordingly. You should be able to solve your knee problem by raising it a cm or two.