But you set it to initial per 1970 Dart specs????
So, in some situations using maniflod vacuum to bring additional advance at idle is a handy way to overcome certain issues with the advance curve.
But if you are using a MP distributor built by Chrysler (as opposed to Mallory) then that's not an issue at all. Set up the way MP/DC says to and it will be very close to what the engine wants for best power under all situations. If its the Mallory built version, we can make it work, but it will take a little more effort. (Send me a PM and we can just do it in person.)
Please see post #19 and the link.
Regardless. 12 to15* BTDC in the range of 600-750 rpm should be close enough not to have major issues. It should be close enough to be tuning it in. There's no reason to be adding significantly more timing at those rpms.
16"Hg is strong at those rpms would be strong. But if idle is at something like 900 rpm, then 12-15* is retarded.