Buzz,
I don't do grouse, but for quail with a 12g, when I did use a 12g, I liked .005 or cylinder in the first barrel and .040 in the left worked well for me. I have a 26" #1 frame Parker with this arrangement (.005R/.040L) from the factory.

I also did very well with another 12g gun with cylR/.005L chokes.