The correct installation of the X-belt on the robot is shown below, first of having the tension lever disengaged, then engaging it.
If this belt is installed around the motor pulley the wrong direction, it will show the X coordinate increasing in the left direction and decreasing in the right direction (which is incorrect).
When the X-belt is installed correctly, then the X coordinate should increase when moving the machine to the right and decrease when moved to the left.
If your motor box is mounted on the back of your carriage, then the belts will be installed the same, but you must go into the settings and check the "Reverse Motor Direction" checkbox and restart the app.