Setting up OpenTX outputs for reverse thrust

Hi, I’m setting up reverse thrust on an E-Flite Opterra using a Pixfalcon controller and Arduplane 3.8.0 beta 5. My question is more about setting up the transmitter for manual pass through using a Taranis running OpenTX.

My ESC is set to 1500us as the midpoint with +100% (2000us) being forward and -100% (1000us) being reverse. I have setup my transmitter channel 1 output min and max to -48% and 48% and set the ppm midpoint to 1750 to use the full range of the Y axis on the stick. This outputs 1500us at Y-100% and 2000us at y100%.

My question is, how can I setup the momentary switch (SH) to change PPM centre to 1250us AND Invert, so that stick Y-100% is 1500us and Y100% is 1000us.

Thanks for any tips!