Vaccumn guage readings are often much more impacted by other than ignition timing,,,vacuum leaks, defective carbs, carb settings, leaky cylinders, plug wires, idle speed, the list goes on.
It is a very rough, but complementary troubleshooting and tuning tool.
Relying on a vacuum guage to set timing is not the method that I would subscribe to. I prefer a digital set back timing light at a minimum.
The point being that you may have issues other than timing that you are dealing with that too must be explored.