A safe place to start with aileron differential would be 2:1 (upgoing aileron travels twice as far as downgoing). That is often a minimum, and you will probably need more than that. As an extreme, the full scale Tiger Moth has zero downgoing aileron when the upgoing is at full travel.

The amount of differential required is a function of a number of things. If your airplane is high wing (you did say, "Cub"), then it will need plenty of differential. If you have extra wing dihedral, you will need even more differential.

Have you thought about mixing in a bit of rudder? Aileron/rudder mix accomplishes pretty much the same thing as differential. If you have a computer transmitter, the mixing function is readily adjustable and you can tune the airplane to be just right to your tastes.

Dick