The car should still run fine with it hooked to manifold vacuum unless its way overadvanced. That aside it will run identical once on the road because the ported vacuum comes into play once the throttle blades are open enough to expose the ported vacuum slot, I.e once it's off the idle circuit. I'd suspect that your damner has slipped or is in accurately marked, or maybe the light is faulty.

For what it's worth my old 273 with a 340 cam liked 15 initial, and about 35 total.

Not to be a wise guy but you're sure you're reading before and after tdc correctly, right? Before tdc is further counter clockwise from 0.