I designed a set of 3 brackets that can solidly attach my calliper to the frame of my Mendel Prism 1.5. The advantage of that is reliability and repeatability of the measurements. It all worked so well that I would need to hunt down a more precise calliper to be able to calibrate further. YES I am saying that I have reached the limits of my calliper which is 0.01 mm. I did about 3 runs of calibration on each axis to get there. Kudos to Josef Prusa for his awesome reprap calculator. At some point you may see that the new esteps values makes your steppers overshoot, I averaged that estep value with the previous one that didn't overshoot and that got me right on the spot.
Basicly you use those to attach a calliper parallel to your axis's or to your filament.
It allows you to calibrate your esteps for the axis and the extruder.
I strongly suggest your visit my blog here for a proper explanation:
*Note: the bracket to attach the calliper on top of your printer (above the extruder is missing, my bad i lost the files) but you will most likely all have to redesign your own brackets since your printer is most likely not a Mendel Prism. Were a rare breed. This was really posted to teach others how precision calibration is achieved, not provide a pre made solution for each printers out there.