Make sure the servo arms and linkages are set as close to equal as possible (mechanically speaking) and then adjust further through the radio's programming. I'm not an Airtronics expert, as I fly mostly Futaba, but I think you should be able to go into the programming and fine-tune your adjustments using the sub-trim and endpoint adjustments.

I think RCKen has one of these radios- Ken, any input on the subject?