Takeoff,

Does your P-51 call for setting up aileron differential? If not I wouldn't concern myself, just setup the control throws as recommended in the model's instructions. Since we are not sitting in the airplane with a ball and needle to coordinate flight its a pain to deal with in getting the differential correct its a trial and error process.

With that said probably the easiest way to setup differential is with dual servos setting the end points adjustment on the transmitter. Setting up differential mechanically is another thing, and far more difficult getting the throws correct but it can be done. You will need review your radio's manual in how to set the end point adjustments. I am not familiar with the DX6i and someone else can probably add more.