Use or don't use vacuum advance on distributor???????????????????

Tell me if this is right. Say it take 15" of vacuum to fully deploy the vacuum advance. As long as you had over that under load (at idle in gear) you would be better off hooking to manifold vacuum. Once the throttle blades open ported/manifold vacuum are the same? however if you have a large cam and low idle vacuum. ported would be better because the timing wouldn't be consistant.