Hi guys,
Sorry i made a typo mistake up there, I meant to say
water/thinner : paint ratio is 70:30 instead of 30:70... that would be too thick..
I've changed the error in my post anyway.
Honestly, i have no idea what PSI stands for. It is used to measure the amount of air pressure coming out from your compressor to the airbrush. Normally, the gauge that measures the pressure can be found fixed on the compressor though there are also external ones.
Just adjust the gauge accordingly.
Sorry i can't help you on the Humbrol paint. I'm sure others can.