Expo is not this complicated....if you find that the airplane is too sensitive then soften the center. No matter what rate you're using.
In other words if your aircraft is flying along and you find yourself "chasing a control surface, it was ailerons for me, add expo. A formula there is not. Start with 30 expo and fly if you like it leave it. It will be better, if you think it can be even better add another 10 expo.
Here's where the arguments begin. I don't believe in expo for learning to fly. If you don't have shaky hands learn to fly with your fingers first.