Expo is used to soften up the stick around neutral to make the plane more easilly controllable on high rates. For JR and Airtroncis, you need positive expo to soften it up, for Futaba and I believe Hitec too, negative. To check, set the rates as the same (set low rates to the same throw as high rates). Then turn the expo up way high positive on one of the rates. Move the stick about a quarter throw. If it moves less on the throw with lots of expo, then you need positive, if more, then go with negative.