Mike
No expert on driven grouse by any means but on the occasions that I have enjoyed most birds are generally killed at under 30 yards' distance from the shooter.
The gun's I use are choked .015 in the first barrel and .005 in the second using an ounce of No.6 or 1.1/16oz (30gr) of No.6.
Cylinder and skeet chokes will work just fine.
I shoot a .010 and .020 over setters and use No.6 cartridges.

Good Shooting
T.C.
The Green Isle