Base timing. Really ?

So im trying to figure out how to start using the vacuum advance.Ok, like i said, it likes 20 intial and around 34-36 total.. So, being that the vacuum advance starts working as soon as the engine fires up, im assuming i can no loger set the initial at 20 because when the engine starts and the vacuum kicks in, it will jump WAY up to whatever the vacuum advance gives PLUS the initial? so, im assuming this..if the vacuum gives 15 degrees of advance at idle, then i should set the inital for 5 degrees so when it fires up and the vacuum advances kicks in at idle, it will have 20 like its supposed to? OR am i all wrong and it actually needs to be set 20 initially to even get her fired up easily?