I agree ukrconsul. 15% should be a good start (provided that the model is not so twitchy that it is too difficult to fly. If so I would reduce the throws a little too).

The trouble, as indicated by david cutler, of using expo as high as 40% is that it can produce a step. Control is lovely and soft around neutral but when you get near the ends of stick travel the servo has to make up for this lack of movement by going to full deflection very quickly.

I would suggest. Get the model flyable with the throws. Forget dual rates. Start at 15% and work up.