Here's a calibration sheet that I made which makes it easy to level a Makerbot Replicator.

This one also has an affordance (the attached square) to make it easy to keep track of where you are in your measurement in case you get lost, as I always do.

Instructions

Print it out without a raft, and use your calipers to measure the thickness of each corner. Adjust your leveling screws and reprint until all four corners are the same thickness. Ideally this thickness should be slightly less than your layer thickness value.