The internal guide sensor on the ST-X is oriented 90 degrees from the main sensor's PA (the sensor is mounted to one side of the main sensor not at the top). For conventional guiding, this requires the guide angle to be rotated -90 degrees. I have patched Adam Block's system as well as Gerald Zima's TOA150 at NMSkies. I have also made an edit to the master copy of AcquireSupport with the change commented out and referencing this issue.
The right way to handle this would be to detect the ST-X in Guider.Camera Name and/or Camera.Name and do the -90 rotation.