Yes the driving test matters. In neutral it does not take much throttle opening to maintain 2500 RPM, that means it is like a Cruise no load when vacuum goes up. Under load of actual driving, throttle will be open more to maintain speed and vacuum will come down. So less Vacuum timing added.
One last thing, what did you do with mechanical springs? If you look at the specs in SVM, the light spring lets the first stage of advance come in pretty quickly, but only about 6 to 10 degrees of mechanical advance by about 1500 rpm. To get the last 18 degree of advance you have to get up about max rpm near 4600 RPM. Not something you want to do unloaded in neutral for very long. If you have all you mechanical advance in too early, coupled with full vacuum advance at 2500rpm in neutral, you would have way too much timing advance for the operating condition.
Racers and performance setup want all there advance in early, but they do not use vacuum advance, or if they do, they use a can with much less advance that only comes in at high vacuum. If your getting all your mechanical in too early, I would dial initial back to a lower initial timing value like 8, and then connect vacuum cannister and take a test drive.
