I have a cap with a lot of coupling...both roll and pitch!! I have a lot of rudder throw also !! I mixed them both out with the computer radio ! I first put them on a permanent mix...but then changed it to a switch. I changed to a switch because I didn't like it on while landing.

The biggest problems with trying to mix it all out perfectly.....is

the amount of coupling is different depending on.
1.....the speed of the plane
2......the amount of rudder deflection

for example....if you mix the coupling out for a fast knife-edge pass using little rudder.... then you try a very slow pass with lots of rudder deflection it wont be perfect anymore!! If you set it up for a slow knife-edge pass then try a fast pass, it will be off again!!

I think what I am trying to say is it is not a linear function and has the variable of the speed of the plane also thrown in to make it more complicated

It is however a lot better than doing nothing at all, which is what you are faced with now