Leveling the Rapman's bed to ensure it's perfectly parallel to the XY toolhead's plane is something important.
Until now, like a lot of people I guess, I was doing it "by eye", trying to achieve a thickness of about 3 sheets of paper (hot-end/bed distance) where the 3 leveling bolts are placed (back left and right corners + middle front).
It usually is enough for classic 3D printing.

But as I allready said, for pen-plotting and especially for milling, it's important to insure that the bed is perfecly leveled. Or at least as perfect as it can be.
So we're going to need a more "scientific" way to make sure the bed is at an equal distance to the toolhead (at least at these 3 leveling points).

And I think there is an easy way...

It took me a lot of time, web searches, trials and errors to find a way to convert a bitmap image into a nice Gcode tool path.
I have tried many things including extruding a greyscale image into a 3D mesh and then extract its first slice using Axon/Skeinforge.
But this method, like many others, produced very poor results...

...until I found out there is a pen-plotter project for the Makerbot called "Unicorn".