Here is a stepwise example solving your knive edge mix problem. There are screen shots attached showing each step.
Enter Menu > Fine Tuning > Free Mixes and press Add.
Set up your mixer and enter Advanced.
Select and enter Curve.
Select Curve type and change to |x|
Select and enter graph of curve.
Select Point 1 or 3 and move up and down to adjust Out value.
Mix Value = Master Value * Out Value
In this example 10% * 100% = 10% for full left rudder and 10% * 50% = 5% for full right rudder.
Check mix in Receiver Output.
Lukas