Please help me with my setup

[Koarl]

Hey guys,

I´m setting up a full house glider on my new Taranis and now I have some questions, here is the first one:
Camber should be controlled by RS, snap flap will be activated by a certain switch, lets say SA-.
For camber and snapflap I need a mix that adds snapflap by the amount of elevator minus the already applied amount of camber controlled by RS.
An example: 20% camber is applied, then 30% (elevator pulled) snap flap. With a simple mix that adds the amount of snap flap to the previous mixer line (camber) the output would be 50%, but it should be 30% instead. The problem is that I don´t want the amount of camber to be added to snapflap.
If I set, for example, the snap flap mix to replace the camber mix, the flaps would go to zero when I activate snap flap while elevator isn´t pulled.
Or how would you setup this in general? I already downloaded a couple of files, but it´s hard to discover how they work in detail.
I hope its understandable...

Thanks in advance.