In the assembly manual (page 33) it tells you to leave a 30mm gap on the left side of the X rail. Then later on on page 36 it says to install the X motor assembly up against the rail. These dimensions do not allow full travel across the 300mm build area. To get full travel across the build area it is necessary to center the rail on the X beam. This would mean approximately 50mm gap on the left and right sides. As a side benefit, this also allows the Y axis cable chain to line up better although it is still not exactly in line.