Sounds like you need to talk to someone who knows how to program your transmitter. It's best to use the innermost position on the servo arm and the outermost position at the control horn. You should be able to set your travel anywhere from 0 to 100% and your mix anywhere from 0 to 100%. I'm not familiar with the 12FG, but I'm sure it has the capability to do essentially the same thing that I did with my JR 9303. Here's basically how my Gripen is set up:

Select Delta or Elevon mode. One elevon is plugged into the aileron channel and the other into the elevator channel. Read your manual, I'm sure it covers how to do this. Put canards on an aux. channel mixed to elevator (you should be able to set whatever mix rate you need). I use two mix functions so that the 3 position switch can be used to switch canards between full travel, half travel or off. For example Mix 1 could be 50% of elevator and Mix 2 25% of elevator. You can also set the end points for the aux. channel to give the desired travel.

I can't tell you specifically how to program a 12 FG, but I'm sure there are others who could help.

Good luck,
Joe