15 BTDC at idle is a reasonable starting point. However, the 007 dizzy should provide something like 17 degrees of advance, not the 10 you are observing, and it should come right back -- no lingering advance permitted. Take it out of the car, take the breaker plate out (mark the orientation), and see what's going on with the springs and weights.

You can pretty much throw out the stock timing specs, as today's fuel is very different from 1968 fuel. A B20 is not original to your car, and who knows if it's a hybrid of several types or not. Experiment, but fix the dizzy's internal problem first.