I would start at -35% on your Futaba--- Remember it just softens the movements of the surfaces AROUND THE CENTER of the stick travel- Full stick still means full throws!
It will make your flying smoother and as you improve you may actually want the airplane to be more sensitive- My airplanes are ridiculously sensitive, to the point that my friends do not enjoy flying my planes cuz they seem twitchy!
I started in 1982 before we had D/R and expo. (At least I did not have it-) and it helped my flying. Now I can fly a plane that is SUPER sensitive without overcontrolling it... Mainly because I learned that the more I moved my thumb, the more the airplane would move
WHat are you flying and what is your experience level?
Take care and have fun!