As a rule it is trial and error. Keep putting washers in until you are happy. I big diameter prop will increase the torque effect, but the lower pitch will reduce it. A mate at my club was using a 12.25x3.75 with that engine on his Hype 3D. That may help things, though you may lose some vertical

But before you do that make sure that the rudder is straight and that is not making things worse.