That's the first time I've considered the vacuum advance coming into the equation. I figured it only upped the timing where there was little to no load, which is usually a safe zone for detonation. Is that not the case?
Plugging up the hose is super easy, just trying to better understand the science here.
The explanation from mderoy340 is right and your conception is right. His advice is to start with no vacuum advance and then work it in after the mechanical advance is all set; sounds like a good plan.
BTW, IMHO going beyond 15 degrees initial in this engine is not necessary; you have all sorts of low and mid RPM compression pressure to start with and peaking it up a bit more is not needed. And no need to go into the upper 30's on the mechanical advance. If you wanted to tweak and tune for best ET's, that is another matter, but no need to go high on timing for cruising around.
