Read the V.M. Starr on Bob's site. His information is much better advice.
[url]
http://members.aye.net/~bspen/starr.html[/url]
It all depends on how you load the gun, what wads you use or don't use, and what kind of performance you are looking for out of that 20 ga. gun. The difference between using hard 1/8" thick card wads over the powder, as opposed to using soft felt wads, can mean as much as 200 fps difference in velocity, depending on which powder you use.
TIP: Use round ball when working up decisions about which wads or cards to use. They can be shot over a chronograph easier than a load of shot. Once you have those decisions made as to components, and powder, it is easy to change the Round Ball for a load of shot, and then work on patterns.