To get a good understanding of the gauges and the pressure/temperature relationship you have to do some reading. Look up superheat and subcooling. When you get these two terms down then you'll be able to figure out any refrigerant and what the pressures are supposed to be.
Originally posted by nlovas
Add 30 degrees from ambient for high side pressure.
Low side temp should be 38 - 40.
Does this rule of thumb work for other refrigerants? i am particularly interested in r134a and r12.
Where can i get an understanding of what the low pressure and high pressure should be? The books i have now are gobbledygook.