Vacuum advance

Im getting ready to start working on my distributor and get my mechanical advance limiter setup. Right now It looks like I need about 14 degrees of mechanical advance to get to a total of 34* i have a ton of questions but im gonna start with this one. I have read and watched alot videos about manifold vs ported vacuum, the book i have (fbo tuning tricks) says to use manifold this is where im lost. So if im using a manifold vacuum im getting about 18 uom at idle let's say I have 15* in the can and im at 15 initial that's 30 degrees at idle. Why not just lock the dissy at 34* and roll with it? Then on the other hand if you use ported the vacuum don't come in until partial throttle. So if you have a total of 34 plus 15 in the can your up to 50* at cruising speed isn't that asking for detonation?