The calibration process 2n2r5 means (I think) refers to the X Y and Z settings in the Firmware EEPROM settings, which you can find in Repetier-Host.
The technique is pretty simple. You print an object of known size (the nickle ring, or a cube like http://www.thingiverse.com/thing:45225), and then measure it carefully in all three dimensions, making sure you keep track of which dimension is which before removing it from the print bed. (Mark the front face with a Sharpie, for instance.)
Then you figure out a scaling factor that would convert its actual size to its intended size. For instance, if it's supposed to be 25mm and it printed at 26mm, the scaling factor is 25/26 or 0.961538. You do this for each of the three axes.
Then open the EEPROM settings and find the X, Y, and Z steps per mm:

Write down the numbers there, then multiply them by the relevant scaling factor, and put those numbers in their place.
Then print again. Rinse and repeat until it's as precise as you want it to be.