Even if you know that your marks are ABSOLUTELY accurate, you'll still have to experiment to find out exactly what your engine likes. Whatever it likes best, that's what the dist needs to deliver, regardless of what the timing marks read.

I've read that some engines like the total as low as 30°, and some as much as 38°. 34-36° seems to be quite common. Some high compression engines must run a lower number than what would actually produce the most power, in order to prevent detonation, on today's pump gas.