Checking/Setting Rear Air Shock Pressure?

So, was reading the FSM for what the correct pressure in the rear shock is and said anywhere from 7.1 psi - 43 psi, but then went on to say NOT to use a tire pressure gauge as it is not accurate and will let air out. It didn't say the right way to check so I am a bit mystified... how do y'all check and set the air pressure in your rear shock for those that have air assisted rear suspension?

Brett

A hand pump with a built in guage and a quick release connector rather than screw-on. Bicycle shops carry them.

A little pricey, but likely a quality piece.......
www.progressivesuspension.com/product/1468/mini-gauge-pump

Thank you much appreciated