I found that is is very plane dependent. 40% or -40% expo is way too much on some planes or control surfaces while others need up to 60% for 3D. It is very easy to tell when you have too much expo. You will end up "chasing" the controls. It is easy to over do it on the rudder. With 60% + of expo the first 1/2 inch of rudder stick movement will give you like an inch or two of rudder movement. The next 1/2 in of rudder stick movement will give you 4-6 inches of rudder movement. You end up either giving too little or too much rudder and then trying to correct causing you to chase the controls. This is particularly evident when hovering.