You can do this on the 9C in Pmix 7 which utilizes curves and makes this fairly simple to do.

Just go into Pmix 7, turn the mix on, make master Flap and then make slave Flap, you will then need to turn Link On, and make this active on Switch X (your choice) in the Center/Down position. On the next screen you will be at the curve screen so in the first and second curve position, I would leave mine at 0% and then in the third position, I made the percentage roughly 10%, in the fourth one I made my percentage roughly 20% and the last one I left blank.

This worked out well for me, you may of course need to change the percentages to suit your application, but I beleive this may be what you are looking for.
Todd