3D Printer X-Y Calibrating Aid

This is an aid to make it easier to calibrate the x & y axes of your 3D printer.
Using calipers to calibrate the x & y axes was a big chore for me since I could never seem to get consistent reference points on the carriage & frame to make measurements. I was resorting to taking several measurements (invariably all would turn out slightly different), and then average the results.
This thing is just a simple pointer that you can clip to the carriage. The pdf file is a mm graph that you lay on the bed.
11-23-13 Update: Made a revised .stl file with a built-in "clip" (meant to mount on a 6 mm thick extruder support). Also made the pointer multiaxis so you can measure both x and y axes without having to move the pointer