Help: mixer for a self centerig throttle

I have a devo 7e with 3x2 extra-switches. I want to try a self-centering throttle. I know the steps for hardware mod. My problem is: how should be the throttle mixer to works like Phantom RC?
When the stick is in mid position it should be neutral.
When i move the stick up or down it should increase or decrease throttle value.
When i release stick to go to neutral position the last value of the throttle should no change.

Thanks

P.S.: Sorry for my English.