The short answer is that there is no such formula. Offset thrust is used to compensate for the asymmetrical thrust produced by a turning propeller. The amount of unwanted yaw varies depending on the size and pitch of the propeller, the power being developed, and the speed of the airplane, way too many variables to be solved in a single formula. It also changes for every flight condition. For R/C airplanes it is usually enough to simply trim out any unwanted yaw with the rudder trim control. If the unwanted yaw is significant, the amount of offset thrust is best determined by trial and error, and as Jack Hyde says, somewhere in the vicinity or two or three degrees usually works about right.

For free flight models offset thrust is an essential part of trimming the flight profile of the model. A typical trim setup has the model climbing in a right hand turn, then gliding in a left-hand turn. This uses a combination of left rudder trim with right thrust. A lot of trial and error is involved in precisely tuning a free flight.