Yes, another timing question!

So, after replacing my dizzy with a MP unit that I can adjust, here's where I am. I hooked up a vacuum guage and from 12 degrees initial, gave it more timing. Naturally, as the rpm increased, so did the vacuum. So there I am at 22 degrees intial and an idle of 1250rpm at 12" of vacuum. Therefore, I backed down the idle speed to 850rpm and the vacuum dropped to 6". Seems crazy high for initial, so I backed it down to 18 degrees at 850rpm and still have 6" of vacuum. I adjusted the dizzy to give 20 degrees mechanical advance and here I am. It runs well, not hard to turn over at all. My queston is, how do I know this is where I'm suppsed to be?? I'm at 18 degrees, but why not 16 or 20?? How do I zero in on this?