timing without dial back.

Going to be useing just my mp distributor at first, when I add a paxton to my car. Any idea of a good starting point on the initial timing for 8lbs of boost. I'm at 20ish right now. I was thinking maybe 16 to start, will this be enough? I will get one of those dial back boxes at some point, but for starters just with what I have now. It's on a small block engine with a 4 inch crank, xe284 cam, rhs heads, air gap, 4 speed in a 68 dart if this helps.