Keep in mind, that is not the proper way to zero your calipers. To do it properly you really need a 1" standard. Something like this
http://www.use-enco.com/CGI/INSRIT?P...PARTPG=INLMK32 . Measure the block in your calipers, holding the same pressure you would when measuring normaly, and move the dial to 0.
The big question is, why did it change in the first place?