I NEVER time by vacuum - particularly on a race engine. Ask the engine builder how much advance should be in your distributor, and when it should all be in. Make sure it has that curve, because any old distributor curve won't work on your engine. Then put on a timing tape (unless you have a degreed balancer.) Rev the engine up to where all of the advance should be in, and set the timing at the spot the engine builder told you.

Example . . . if he says 36 degrees (and I'm not saying 36 is the correct number - just an example), and it should all be in by 2500 RPM, crank to 2600, set it at 36, and let the initial fall wherever it wants to. In your case, it should drop back to 15.

Initial timing means squat on a race engine. It's the curve and the max that makes it all work.