You will need to know your total mechanical advance. Do you have a dial back timing light? What heads do you have? Tranny, stall, rear gears, etc. What is it set at now with vac adv unhooked at idle? You could just increase at 2 degree increments until you hear rattling under load and then back off until it goes away. Best way is to experiment at a dyno or the track. All engines will be different.