I haven't shot any 20 ga round ball, so can't offer any specifics, but have used a lot of them in 12 gauge.
You can use shot data for the appropriate weight, just substitute the round ball. I use cork wads in the shot cup to adjust height if necessary. I've even dumped the shot from Federal 1-1/8 oz (12 ga) field loads and replaced with a round ball, these work really well, and I wouldn't hesitate to use one on deer (where legal and appropriate, etc.)
I generally use wheel weight balls, not pure lead, and use a ball size that fits inside the shot cup. Fed or Win shot cups work great, Remington not so much because of the rib on each petal.
