Ok, so it's reading inches of mercury vacuum. 0" hg = 14.7 psi map, 30" hg = 0 psi.
Vacuum Pressure - Converting Units (compare PSI and Inches Mercury Gauge)
You want to accelerate around 5 on your gauge (but not below 2-3), at the border of Poor and Fair. It sounds backwards, but your engine makes power more efficiently with an open throttle and low rpm. Make power and accelerate efficiently, then switch and keep the gauge high for cruising. Don't go above 2500 rpm or so. I prefer shifting at 2000.