Are you using ported vacuum or full intake vacuum on the advance? When coasting, you should have little or no advance from the vacuum canister if the port is above the throttle plate. Someone has BADLY misinformed you about how a vacuum advance works. It adds about 8-10 degrees of total spark advance (crankshaft degrees) at light throttle cruise, and drops that advance off during open throttle pulling, and usually when coasting with the throttle plate(s) closed. "Ported vacuum" comes from a fitting slightly above the position of the throttle plate at idle, so it's possible your idle speed is set a bit too high if you're getting vacuum at the distributor while idling.
Jerry
