ok, measured my martelli mat with three different brand 6.5" rulers. The seems to lay perfectly at that measurement...but within the rulers the martelli 1/2" measurements seem to be just to the left of the cutting lines on the rulers.
Were I to use those lines I think my cuts would be about 1/16" off each cut, on the too big side.
But as I said I'd never use mat lines to cut with.