[mr0077]

First, make sure you're taking the vacuum off the correct port...second, under no-load conditions (just revving up the motor while sitting) you will get the maximum available vacuum advance, since the manifold vacuum will be high. Under load the manifold vacuum will be lower, maybe varying from close to zero at WOT to 12-15 inches of vacuum under cruise load, (depending on a lot of things).
Do you have an adjustable vacuum pot? you might drive it gently and see if you get any rattle or ping...as you open the throttle the vacuum drops, and the vacuum advance with it, so when accelerating you get no or very little vac advance. You just want to be sure there is no ping under any load/rpm conditions with the gas you intend to use in your car.
Vac advance helps cruise mileage, but won't do anything for max power.