OP is at cruise where it is likely ALL of the advance will be fully deployed. I am concerned that he might be somewhere over 52 degrees or so. He might be firing too early at cruise conditions. High cylinder pressure well before the piston gets to TDC might not be the cause here but if we ask, we might learn more about what we are looking at.
I understand that, but it won't be fully-deployed at cruise, as there's still a load on the engine at cruise; peak vacuum only occurs under two conditions - at idle in neutral, or at cruise when you take your foot off the gas. Best way to determine what the VAC is doing is to connect a vacuum pump to it (MityVac) with a timing light hooked up and see how the timing changes as varyng levels of vacuum are applied. To properly spec a VAC, you need to know four things:
1. At what vacuum level does it start to deploy.
2. At what vacuum level is it fully deployed.
3. How much advance does it add when fully deployed.
4. Engine manifold vacuum level at normal idle.