A good mumber to start with is 10%. there would not be a lot of effect at that. Go up or down from there Nothing is worse than having too much of anything to start with and having to fly for a while with it. On a JR transmitter the more expo required the number is positive. I understand on the Futuba transmitter the more expo you want the number is negative. I am refering to here to dead at centre stick. it is possible to reverse that.

Expo does not replace dual rates. You can still have too much movement even with expo. If you are looking for little movement at the centre stick position, all that movement will come in at the stick extremes. That can give you a surprise. So be carefull with getting rid of rates and relying on expo alone.

Ed S