Following is the official updated version of how to install a y-belt onto the carriage of most machines.
If you are getting a y-coordinate reading that decreases when moving your machine from back to front, then it could be that your y-belt is installed reversed.
When the belt is installed incorrectly, the belt goes around the motor box pulley the opposite direction. For instance, in the WRONG installation, if we pull the machine forward, the motor box pulley will turn clockwise. However, with the CORRECT installation the motor box pulley will rotate counter-clockwise.