@gianluca Magnani Since this is your first calibration, you might want to take a look at this guide: http://www.reprap.org/wiki/Triffid_Hunter%27s_Calibration_Guide. It’s very detailed, with solutions for specific problems.
That said, I agree with Greg – 100% accuracy is not required. I had to do a little sanding on the roller and middle z pieces to get bearings to fit, and I made a wedge that I applied with a c-clamp to open the corner blocks to fit the tubes in. I am very happy with how my MPCNC works.