I completely understand. You want this to be as accurate as possible. We can assume the Marlin firmware is dialed in, as it seems to be working on many MPCNCs. In my case, one of my machines prints the 20mm hollow calibration cube (http://www.thingiverse.com/thing:271736) at about 19.6 x 19.8 x 20.1. That’s pretty darn close, but I assume it’s due to backlash that I still have to work out of my system.
Just for information, you might want to check out an interesting guide on calibration at http://reprap.org/wiki/Triffid_Hunter's_Calibration_Guide.