I set it to 20", and played around with it to see what works best, and went back to 20". Anything less and the brakes didn't feel right. But since you don't have a booster you may not need as much. I also bought a vacuum pod through my work (Oreilly's) to hold a little reserve for the shutoff switch. Everything works great, except I think I have a small vacuum leak in the hvac control unit, as it'll lose vacuum if it's in the "off" position, but won't if it's on heat, a/c, defrost, etc.
I'm also running a separate pump for the transmission, a cruise control pump off of a '99 or so Ford Super Duty, which is doing that job nicely and doesn't seem to care that it has to run all the time.