Green Dot is too fast burning for the 20ga with heavier than the suggested 3/4oz loads.
Use Hod. Universal for a good economical powder for the 20ga. It works for up to 1.0oz loads with the 20ga and is superlative in the 28ga with 3/4oz target loads.
Leave the Green Dot for the 12ga with 1-1/8oz and 1-1/4oz low velocity loads. My personal favorite load for the 12ga is a 3-1/4dr.eq. load and 1-1/4oz. Back before lead was out-lawed for waterfowl shooting, I used Green Dot and 1-1/4oz of #5's for shooting ducks. It was efficient and deadly to 50+yds. Worked much better on those below freezing mornings than HS-6/Win540. Much lower powder charge than BlueDot.
For cripple's I'd substitute #71/2's, and use #8s for doves..... One load did it all.
Ah, the good ole days !!!!