My set up is like this. Ch 2 is aileron, Ch 3 in elevator, and Ch 4 is rudder. I have a switch programed to activate two mixes simultaneously. I throw the switch, then as long as the switch is "on" my elevators (Ch 3) move slightly up (to correct down pitch coupling) based on my programed % of rudder (Ch 4) throw. I just program for example 3% of rudder throw as the starting point. When ever I add rudder (Ch 4), then 3% of that degree of throw is copied by the elevator. Then I just add or subtract the "%" until the plane goes straight. Usually a different "%" is needed for each direction. Maybe 3% for right rudder and 4% for left. It also could be drastically more. Roll coupling is programed out the same way. I mix Ch 4 (rudder) as the "master channel" to Ch 2 (aileron) as the "slave channel" I start out with a guess (5% or so) and then add or subtract "%" until the roll does not happen each direction. These mixes will only be truly exact if the same throttle setting and rudder input is used. Meaning for high speed knife edge you may need a different "%" of mix than at low speed at a higher angle of attack. Most times the mix will "help" with controling coupling at other speeds or deflections. Some radio systems offer "multi point mixing" which allows you to mix corrections for many stick inputs and always maintain the straight line no matter the deflection. I hope this helps.