I believe timed vacuum is load sensitive.
Putting a load on you engine will decrease your engine vacuum?
Vacuum advance is for better fuel milage & other things.
The 20 degrees is just as a general rule of thumb.
You can lock in 32 degrees or more if you want to, as long as it will start & does not detonate @ lower RPM's.
You can do away with the vacuum can altogther, if you are not concerned about milage.
Basically if you run full engine vacuum @ idle you get full advance. As soon as you crack the throttle your vacuum drop which also drops off your timeing advance.
If you use ported vacuum, you have little to no timeing advance at idle,,,,but as soon as you crack open the throttle you start getting vacuum to the vacuum can,, then it will start to advance your timeing.
