I keep a piece of hefty ground bar for setting up chucks, measuring runout etc. - about 8" of 2" bar, still shiny! I needed to check the quill/table sqareness of a mill, so indicated the bar in the 4-jaw (at two distances from the jaws to be sure it was concentric), faced the end square and - important - recessed all but the outer 1/8" so it *couldn't* be convex - ran a "tenth" DTI mounted in the drill chuck down it and went and found some shims for the column foot and the forklift to hold the head... Shouldn't have bothered!
It now has my initials stamped in the recess, to keep it in the workshop!
Dave H. (the other one)