First off, it may be worth waiting for 1.03 to get released later today. I know that the UK is ahead of us, so it may not be until the evening that you see it. The new update will include a new calibration pattern that should be easier and more accurate than the current one. I’ve been trying to follow it’s progress but I’m sure it will all make a lot more sense to me when I’m actually running the procedure myself 
That being said, I can try and help you through calibrating the machine. I use a manual process myself because I haven’t had much success with the automatic routine.
Did you notice any sag in your tape measure? If not, then I would start with that measurement. Maybe it’s the steel fabricator in me, but I feel like I trust my tape measure more right now. That’s subject to change with the new update. With that measurement plugging into GC, cut (or draw) a 4in (~100mm) square into a scrap piece. Check the X and Y legs of it to see how far off the measurements are from one another. Ground Control has a simulator which will help you figure out what variables need to change in order to dial the machine in. In a lot of cases, in my experience, tweaking the distance between motors is how I get the machine within tolerance. So you ultimately may find that your measurement will end up closer to the machine’s measured distance.
Do you have a linkage kit, the ring, or the stock brackets? That also has a lot of an effect on calibration. Quadrilateral (stock) kinematics are a bit more tricky, and you have to tweak with the sled mounting points a little more to get the machine calibrated. Triangular (Linkages or ring) kinematics are quite a bit simpler, as you only have to tweak the rotation radius.
From what I understand, that isn’t currently implemented, but is under development. Discussion thread here. There’s a link to a Github pull request at the end of the thread which leads you to a more thorough description of the updated calibration routine, as well. Not sure if chain sag calculations are being included in today’s update, but it would be sweet if it was! 
You and me both. o.O
I dove a bit into the wikipedia rabbit hole and I’m not sure I actually absorbed anything. I now know a little bit more about the existance of hyperbolic functions, though! xD
Makes me really appreciate the amount of work being put into the chain sag calculations!