What pressure should my filter pressure gauge read?
There is no single pressure for every pool. Pressures on clean filters should run between 10psi and 20psi. The length of your plumbing, type of pool cleaner and number of valves will determine your clean filter pressure. Turn the pool cleaner off and turn the pump on. Bleed the air out of the filter with the air bleed on top of the filter (when water squirts out in a steady stream the air is out), close the air valve and read the pressure. This is your clean filter pressure. Turn your pool cleaner on. Depending on the type of pool cleaner you have, your pressure will either go up or down. This is your clean filter pressure with your pool cleaner operating.

When should I backwash or clean my filter?
Backwash your filter (cartridge filters - remove and clean the cartridge) when the pressure is 10psi higher than your "clean filter/clean filter with pool cleaner on" pressure (see "What pressure should my filter pressure gauge read") or every 90 days, whichever is first.