Aileron differential is way to go if you have one servo and channel per aileron and radio with ail diff function. Start with 3 - 5% mix. I am not sure but I think, by using a y harness, you are limited to mixing opposite rudder. With a high wing / dehidral trainer, you may need quite a lot of mix, and that can interfer with normal control, like maybe causing too much drag, which may cause a stall/spin crash. I would start with 10% and adjust as test flights indicate. Be sure to put the mix on a switch to turn it off if necessary, and to be able to experiment with different flight situations. Go to NSRCA Trimming Chart to help you understand all the different variables. Consider all this as an experiment where you will have to try several different settings before finding the best one. Using premixes to fix aerodynamic design problems can get complicated. But it is one of the things that make the difference between average pilot and great pilot.