A quick help for this is to use a second bearing on the top of the x carrage. (Note. my kit came with a spare bearing for this) then use thingvers model "Printrbot LC X-Axis Stabalizer" thing 42480 to hold the two bearings together on the carrage. The use of two bearings will reduce the available x travel to about 5.5 inches, but that is also the distance between the bed mounting screws, whichby jheacock - General
The formula will get you close to the correct calibration, but does not account for small changes in the effective belt pitch as it bends around a small pully. Now that you are close try: new_steps = current_steps * expected size / actual size To compute a new steps per mm for your next print. Good luck getting the error down to a reasinable tolerence and repeatability Jby jheacock - General