Can't help with the screw size, but IMHO, that's got nothing to do with the "creep" you are experiencing. Remove the trigger assembly from the stock and POLISH the tops of the "knives" that come into contact with the trigger sear. Use crocus cloth or a fine stone. That should cure your problem. Do not remove material beyond polishing (i.e. don't reshape AT ALL. If there is a sharp corner, leave it sharp). All you need to do is get rid of any imperfections that will cause "drag" as you engage the trigger. Performed this procedure on both of my GPR's and have no trigger creep.
If I knew you were standing next to me at a shoot, you wouldn't be getting this information. I need every advantage I can get . . . ask me how I know . . .