Yeah, calibration doesn't work for me either, but I just did a couple test moves of around 100 mm, measured with my caliper, and adjusted the XAxisScale (and Y and Z) appropriately.
After a few trials I ended up with 8.0110 steps/mm, but thinking about it, I realized that (400 steps/rev) / (20 teeth/rev * 2.5 mm/tooth) = 8 exactly, so I just used that, and it seems to be the most accurate after further testing. Z was 320, as I used metric m8 rode with a thread pitch of 1.25 mm/thread, so it's just 400/1.25=320.
I've got 400 step step motors driving 20 tooth pulleys on 2.5 mm/tooth belt; you may need to change it for your particular setup.
Cheers!
Wade