Mini Calibration pieces! For checking how level your build plate is

Hey guys! I'm in charge of my school's 3D printer and recently we've been having a lot of problems with leveling the build plate. Turns out that our rubber pads have worn down and the build plate has been coming out of alignment. Currently, we are using a fix seen here (http://www.thingiverse.com/thing:43050) until we can get a more permanent fix, but until then, we are having to level the build plate quite a bit. However, sometimes I level it and I think I have it right, but then I print someth