Your elevator to brake mix can be turned on and off by a switch if you choose to program it this way. Just make your retract switch control the mix. Gear down: Mix on. Gear up: Mix off.
If you are using flight modes just skip this mix completely in the "flying" flight mode.

For the mix itself I recommend the brakes to come on as soon as you give down elevator. Then you will have little or no "tail lifting effect" from the elevator, even at high speeds.