Maybe there is a better way to do, but this one works fine...
I'm using channel 8 of the radio and the following switches : SW8 (2 positions) and CTRL9 (3 positions)
In the main menu, navigate to “controller settings”
Go to channel 8 and activate the CTRL19 (3-pos switch) as shown here :
Then adjust the following values (+32% and +32%) :
Now back to the main menu, choose “Free mixer” and set the following values (i'm using the third mix in this example you can choose M1 if you want). First S => 8, then select the SW8 (2-pos switch) and then go to the arrow on the right to set the travel and offset.
Set travel and offset according to the following screen :
And that's it you're ready to go... I have the following values, which works fine.