When the OP asked "is there a formulae for calculating Expo" I just asssumed he was asking if there was a formula the transmitter uses for expo, which it obviously does, part of its programming. If the OP was asking if there is a formulae to work how much to set expo too...its up to your personal preference. The way I set mine I dont need dual rates, even though I have maximum throws on my Pitts (45 Deg deflection), if I need to make small adjustments to a control surface in flight I can do it comfortably using what I have set for expo, and still have huge deflections in manoeuvers. I fly a wide variety of planes with different amounts, and to me I just adjust the settings until it feels natural for the plane I am flying. I also use a different expo setting for up and down elevator.