Hook up a timing light to #1 plug wire & see when it's firing, on your timing marks.

Should easily crank anywhere between about 8 & 15° BTDC. Back in the old days, I idled most of my Pontiac engines on about 13°. Some engines may like a little more or a little less.

For high performance purposes, your total advance is more important. Most(not all) Pontiac engines like about 34-36°, at WOT. Many Pontiac guys will set up their dist to idle where they want it, start advancing at just above idle speed, and reach total advance by 3000 rpm.