Find center on a tapered, curved rail
Drill a pencil-size hole through a 3⁄4 x 11⁄2 x 11⁄2 " block, cut a V-shape notch into it, centering the notch in the block. Glue a small bubble level on one edge, and insert the pencil, as shown. Working on a flat, level surface, with the pencil pressed to the rail, run the V-block along the rail while keeping the bubble centered. The resulting centerline follows the tapers, bends, and curves of the rail.
—Ted Sheroke, Medina, Ohio