thats a good number for the low side but can be higher if it's a hot day the high side should be about 150-250 psi thats the reason you need a set of gauges as the pressures tells the story of problems suches as negitive pressure on the low side means a bad expansion valve usally and if both low and high side is = then your compressor is bad...

it's normal for the pressure to be 90-100 psi when the a/c is off...