I've been shopping for kitchens (yes SWMBO has finally persuaded me that our 20y old kitchen is in need of some refurbishment) and got chatting to the designer guy in the shop about stuff such as CAD (as you do :)) and the use of granite worktops as a base for aligning things (he's a model RC builder). He reckoned they're pretty flat (at least by by his standards) and promptly offered me two 12" square offcuts of 1.5" thick granite worktop which I couldn't refuse, much to the annoyance of SWMBO ("I can't believe you did that!") :D

A quick test with the edge of an engineers square (about the only thing I have close to a true straight edge) suggests they are pretty good although I can see daylight in places between the edge and the top face, but how could I measure the flatness?