You'll never get an exact prediction for a specific size (unless you manage to find someone with literally the same setup) because even for the same size among different manufacturers, the tires will not fit the same
For example a Michelin PSS and a Bridgestone S04 both in 245/40/19 will fit differently on a 19x9.5 wheel. This is because they have their own designs, and the tire's shoulder will be more or less rounded or squared off depending on the manufacturer. Some tire companies/models run "wide" while others run "narrow" by a bit
For a more or less "square" shoulder, i think a 325/25/20 is right on the money for a 20x12 wheel. A 305 is only 20mm less so you'll get a slightly stretched look (rim edge exposed by tire).
This isn't a "problem," so much as aesthetics.
Some people freak out when the tire isn't completely covering the rim edge, but thats just them.
Others have the entire freakin' mounting surface exposed in a hellaflush setup.
And there's everything in between.
I think your sizes are fi"ne, assuming you are running a 255/35/20 and 305/30/20
