Having quickly got frustrated at attempting to iteratively level the bed, I decided to create a calculator to do it as a single stage.
Primarily this thing is the calculator, but the decimal knob makes it easier to figure out the number of turns correctly, and the printable gauge is a good option if you don't have a dial-test-indicator.
EDIT: Not having realised how popular the gauge would be I used my original 'rough draft'. I've now updated it to a parametric version
EDIT: And modified some defaults and added a (paper) printed scale.
EDIT: 29/5/13: Updated the spreadsheet with a 2nd sheet for 3-point bed levelling
EDIT: 23/10/14: Updated the spreadsheet to improve accuracy and simplify levelling with caliper/depth gauge for the 3-support sheet