When does vacuum advance kick in?

Got a 408 with a Holley sniper. I’m setting up the ignition, getting my start, idle(18deg) and wot(35deg) dialed in. I never had vacuum advance with a carb but can program it with the efi. Right now I have 40deg set in(still low I think but I’m increasing and seeing how it responds),but at 55 mph/ 2400rpm cruise timing is about 32deg. If I go up to 65mph cruise the timing goes up to 40. I can adjust it, but not sure where it should go to the 40. Is it dependent on rpm, MAP or what, and where would I see it advance to take advantage of it?