Anyone know how to set up a Groove fixture?

While I have been building bikes with a LCFF we also have a Groove fixture for our titanium 11ants frames. There is some documentation showing how to set up the wheelbase, angles, chainstaylength and front/forkheight but I just cannot figure out how to set up the length of the frame. Which datum points do I need to measure? The documentation doesn’t tell, or I might miss the page? Anyone have experience with this fixture and can tell me how it works?

No one here has any experience with this fixture?