Horizon UMX Servo Range

tl;dr: For Horizon UMX planes and an OpenTX transmitter, set output channel to 80%.

From RCGroups:

All Horizon Hobby Ultra Micro (UMX) models, from the very first Vapor nearly ten years ago, use linear servos. This includes the Champ.

These servos are not designed to go beyond limits corresponding to 100% in a Spektrum transmitter (1100-1900uS pulse width). If driven much beyond this range, they may suffer mechanical or electrical damage as their travel is physically restricted. Horizon issue strict warnings not to set the transmitter travel beyond 100%.

For the Taranis and other transmitters that use OpenTX or ER9X, 100% corresponds to 988 to 2022 uS. Thus, it’s essential to restrict the travel to about 80% (actually it’s 78%, but the servos have enough leeway that 80% is close enough).

You can do this by setting the weight on channels 1-4 to 80% in Mixers. A better way, however, is to set the output on those channels to +/- 80 on the Outputs page.

If you have more than one UMX model, set up one model this way, then copy it as many times as needed, changing just the name.

The arithmetic is as follows:
Spektrum_Transmitter: 1500 - 1100 = 400uS for full travel.
Taranis: 1500 - 988 = 512uS for full travel.
400 / 512 = 0.78125 I.e., 78.1%.