You need to understand that servo wheels, due to their rotational movement have negative expo built into them, that is they are twitchier around center than and the end of the throws. Try 20 percent, that will give you a linear feel, any more than that and they will be softer in the center. For Futaba you use a negative expo number to make the center softer so you would need -20 percent, on JR you would go positive to achieve the same effect.