To limit the vacuum advance arm travel, I set my initial where I want it, use a timing light and a mighty vac hand pump to pull in an additional 10* and note the amount of vac required for the 10*, pop the cap off, pump the vac to noted amount, mark the vac advance arm with a sharpie right where it passes through the housing, pick a appropriately sized short roll pin and drill the correct sized hole through the arm where you marked it. Drive the pin in and you now have a mechanical stop on the vac advance.