With ported spark, initial(base) timing is the same static as it is with the engine running, as the vacuum pickup orifice in the venturi for the vacuum advance is above the throttle plate at idle, thus not exposed to manifold vacuum; when the throttle is opened even slightly, that orifice is then exposed to manifold vacuum and the vacuum advance comes into play.
Base timing spec on all ported-spark Chevrolet engines (1969, for instance) was between TDC and 4 degrees BTDC; the only exception was the L-88 (which had no vacuum advance at all), at 12 degrees BTDC, and several 327/210's in Camaros and full-size Chevrolets were actually spec'd at 2 degrees AFTER TDC. This put all of these engines (except the L-88) WAY retarded at idle, and with ported spark, they had no help from the vacuum advance at idle.
To make up for the retarded idle spark, most of the distributors on these cars provided between 24 and 38 degrees of centrifugal advance, to bring total timing back where it belonged for high-rpm performance. Later cars (when catalytic converters and HEI came on the scene) had base timing bumped back up closer to the 10-14 degree level, and the distributor centrifugal advance mechanisms were cut back to 20-24 degrees, keeping total timing at about the same level.
Until catalytic converters and electronic fuel injection came along later, it was a constant game of calibrating base, centrifugal, and vacuum advance spark timing to match up to the Federal emission testing procedures; California procedures were different (and still are today), so California engines were calibrated differently than 49-state engines, just to pass the California tests (remember the 305 LG4 California Corvettes in 1980 that got a $50 credit?). Remember the infamous "electronic Q-Jets" with the duty-cycle solenoid-operated metering rods?
Lots of weird things were done to carbureted cars to get them to pass emission tests, and "ported spark" was one of them.
