All of my 20ga trap recipes are for 3/4 oz 7 1/2 and use Green Dot in Federal Hulls with 209A Federal Primers. Load came from Hodgdon site. 20/28 is on my shelf but I have not rolled any yet.
Be wary of using Green Dot in subgauge tubes, regardless if it is a recipe. 20/28 is going to be one of the great powders out there.
20/28 is a great powder, Winchester Super Field (WSF) has better stack height in AA hulls. Longshot is good.in the AA hull.
20/28 powder with CB-1078 wad, Win primers, AAHS hulls, 8.5 shot. WFM and everything fits just right. I've also used Unique powder with these same components but prefer 20/28.
I've used Unique for more years than I care to admit! It's also great for 12 ga handicap and 28 ga. I also like not having 4 different powders for different gauges. It's also still made here in the US, and is less expensive than many other powders.