OK, my mistake. I somehow forgot you were using an 8U, and gave you the setup for a 9C-series Tx (brain misfire!).

Set up the flaps on one channel. To operate the main flaps channel via a switch, you'll need to use a pmix with offset as master and the flap channel as slave. Set it up for the switch and travel you desire. Use the next pmix in the list to set up the second flap channel with the main flap channel as the master. Set the % of movement of the slave channel so that it moves with the master channel. Use a third mix to turn off the knob per the FAQ.