Eric,
Thanks for the squaring tips. I generated a file to cut an "L" into each block vs pins (potato / potahto).
After about 30 minutes and 3 block adjustments, my 9660 machine is reading about 107.6875" +/- .007" either direction. Honestly cannot see a difference with the naked eye, and tension of tape seems to be the biggest variable in the equation at this point.
Cannot believe I got away all these years without doing this.

Previously had a sweet spot at center of table where the gantry settled in to square very, very well (+/- .030), so it got parked there on shutdown. (However, I've had a few zingers come off the machine...)

Now, my stop blocks are tweaked and I'm finally doing it the right way, and will continue to test periodically. Consistency is king!

Thanks again, and I strongly urge anyone reading this to make time for this tweak!

Jeff