I would still argue that the correct measurement should be from the bottom rails (that support the base plate) to the underside of the middle rails.
That way the "Y" rails are better ensured to run parallel with the base.
That in turn is more important mechanically than their relationship to the top rails (which doesn't actually matter at all).
As part of the process of changing out all the L and T plates to aluminum and removing the front middle rail (again thanks for your bracket design! ) I did try using a piece of scrap wood as a spacer to establish the distance between the remaining middle rails and the base.
It simply couldn't have gone smoother or better.
I would highly recommend that as a setup method instead of measuring from the top rails.
FT should just include a pre-cut piece of PVC sprinkler pipe as a spacer for that purpose.
It would cost perhaps $0.50.
But that is just my opinion......
Also, there is still an error in the parts count for the FT5 pieces (should be two, refer to pages 6 and 10) and unless something has changed since I received my kit, there is no open source software attached to the underside of the base and the firmware was not pre-loaded onto the controller card (refer to page 69).