1 kg/cm2 = 14.22334331 p.s.i. and 1 p.s.i. = 0.070306958 kg/ cm2
since 1 lb = 0.45359237 kg and 1 in = 2.54 cm
( Hope I did not make a mistake on the first line, or shame on me )
Now there is no ideal pressure as it depends on :
_ paint ( or whatever medium you use ) viscosity and subsequent dilution
_ what airbrush you use ( big plough workhorse or detail airbrush mounted on fine needle/nozzle configuration )
_ what you intend to do ( large overspray or WWII Italian spaghetti)
_ distance between the airbrush and the model
It may seem very PC but all is a question of trial and error. I saw in a Kalmbach book that a known modeller used his paint pure and sprayed it at high pressure ( by the way, chaps here who make T Shirt airbrushing will tell you waht pressures they use-I'm scared as I hear the figures) and another made the opposite, working on transparencies and building his tint with a lot of coats ( a pretty French world for this : glacis ) so it is up to you : experiment is a part of the pleasure