Hey Tom,
measuring this does not ensure rectangularity. The manufacturer’s assembly instructions do not include a safe method to ensure the rectangularity of the machine. Many users have already noticed that after assembling, their machine was not “square”. Your machine could be a parallelogram instead of a rectangle. This can be checked with a method like this.
Adjusting this would usually require to unscrew the rear or the front feet of the machine and repositioning them a little more to the left or to the right.
You can also avoid problems with rectangularity prior to assembling by finding and marking the correct locations for the holes for the feet of the machine by using one of the geometrical methods described in the thread above.