Well, I'm guessing your tiger moth is a park flyer and has a flat bottom wing or maybe an under cambered wing. Therefore it would usually require down thrust. 2 or 3 degrees. Also , it will require that the engine be offset slightly to the right, say 2 or 3 degrees. These are just sort of rules of thumb. They are good starting points. Don't be surprised if your model turns a little better to the left. That is normal. There is torque, precession, and be cause your park flyer flys so very slow, I would guess that it is flying a little nose up, like it is struggling along a little. This introduces something called "P" factor. The right blade, (facing forward) , generally refered to as the "descending blade" will have more apparent pitch then the left blade. The result is an asymetrical thrust causing a yaw to the left. It helps if your turning left, it hurts if your turning right. I hope that makes sence.