The right pressure is the one that works the best for you with your airbrush and paint. Some airbrushes need more than others. Bottom fed airbrushes generally need a little more than the gravity fed type. External mix airbrushes usually need more than internal mix. Thicker paint needs more to spray well. Detail work is usually easier with lower pressure than needed for general coverage. It doesn't matter what the gauge says if you adjust pressure to get the output you want. It could range from 8 to 40psi. Once you determine the sweet spot for you, the gauge makes a handy reference, whether you set it with trigger up or trigger down. For my Badger 155 with a 1/4oz. cup and paint thinned the way I like it, somewhere around 15psi works for me.