What is the correct amount of vacumn advance?

C130 you hit the mark I was waiting for. Just before spark knock is usually the best for mileage and power. Set with the vac disconnected first. Reconnect vac and try again. If you get ping you can try a vac pot from another engine or a delay valve. Old Jeep Waggys have a ton of them. Color coded. Usually the truck or/6 will have less total vac adv pots. If you have a good parts store guy he can look up the specs in the application/crossover catalogs under the counter collecting dust. I collected the old cats when I worked at a parts store. Have the GP Sor ones if you want to try that approach.