Good info Phil! .....but I tend to believe that a "Miller" would work with a millstone -- grindstones were used mostly in the sharpening and honing professions.
While visiting a medieval castle armory, a tour guide in Germany explained it the following way. In those days traveling bladesmiths would cross Europe in wagons moving from town to town to repair and sharpen cutting instruments (scissors, knives, sickles, chisels, planes, etc..) and they would carry portable foot powered grinding stones mounted on stands where the baldesmith could sit right behind the stone. To keep that mass from moving and jerking, they would have to sit very close to the stone while it's rotating in order to grind and hone a good edge. It took a lot of experience; hence it was a learned trade -- young men went from Apprentice, to credentialed Journeyman, and eventually to Master grinder. Huge amounts of concentration and constant weight shifting was necessary to keep the entire rig stable, and that meant that they had to get their faces very close to that stone -- hence the popularity of the phrase.
