How are you controlling the function? What actual function are you using?

In the Butterfly function, in the Elevator setting section, you can set the curve. I set offset to -50, Rate A to +50 and Rate B to +50. This put the elevators centered when the stick was in the "butterfly closed" position, and then moved elevators as the butterfly stick was moved. The second page of the elevator setting, where you set the rates for ele and ele2, is where you set how far the elevators actually move when you deploy the Butterfly function.

if necessary, once you set the offset, you can adjust the intermediate movement of the elevators by using the point curve. That will allow you to exactly "dial in" the elevator movement for each intermediate position of the Butterfly.