72 360 first start question

Correct, the slit is the slit in the dampener and BTDC is before TDC. Yes when the piston is at true TDC the slit needs to be at zero (TDC) on the tab & a slipped dampener or a mismarked dampener is somewhat rare. Just wanted to get that out of the way for clarification. 15 BTDC is a VG ballpark initial timing setting & turning the slit to that is a very close correct initial timing setting then getting the magnet dead even with the tooth and the rotor at that point should be pointing fairly close to the cap terminal. 0.0632683" is (1) degree around the dampener circumference so 15 deg would be .949" if you have some calipers and or a mike or even get close with a tape measure & cut a strip of white paper & use it as a ruler to mark 15. EDIT I reread your post & you found true piston TDC with the stop or the dial indicator and at that point the slit ain't at zero on the timing tab? If not how off was it for curiousity. We gotta get that straightened out first