I'm not sure how much of an advantage this provides over using a hanging scale like I have seen Ian use.  But at the very least you don't need good eyes for this one.   

All I know is this is my preferred method for calibrating my y axis belts.

Also I was on the phone with Solidoodle Support a few months ago, and I told them about the method.  The customer experience manager's reply (yeah I had a problem that bad) was "$10-$15 you say?... how'd you do that again?" like he was taking notes. At which point I told him I would make him a video demonstrating it. his reply "Oh that would be great!" I'm hoping I contributed to there QC process;) Or at the least helped come up with a solution for other customers belt tension issues. 

regardless it was a funny phone call.

Anyhow I posted it on the solidoodle FB page a while ago, but I doubt it was well seen. So I figured I'd post it here as well just in case anybody could benefit.

With no further adieu:
http://www.youtube.com/watch?v=S3RJ92NZ2as