I forget, does the DX6i have a 3 position switch for flaps? If it does and the gear channel is proportional (it's not on some radios), you can make 6 the master and 5 the slave for the mix. If not, you'll have to settle for an on/off setup for flaps. You definitely don't need takeoff flaps for an Avistar though. As stated before, you really shouldn't use the flaps at all at this point in your learning. They'll add some fun to your flying in a couple of months once you are soloed and have your landings looking good conventionally, but as a beginner you will just add another complication to a system that already works beautifully if it's set up right.