You don't use end point adjustment to set rudder servo throw, you use the gyro limit pot.

For the trex put the gyro in rate mode, center the tail pitch slider, rotate the arm 90 degrees at a time till you find the one thats straight up and down. Use the closest hole to the servo center.

Adjust the servo position and or linkage length to center the tail pitch slider the use the limit pot to get the most throw you can without binding.

Set it to heading hold and go fly. Use the rudder end points to adjust the piro rates, you can go over 100 and you may have them slightly different to get symetrical piro rates.