You've miscalculated while trying to find a workpiece's center. Solution: Eliminate the chance for errors in math by using a measuring tape with a half-scale on the lower level. This board measures 29 3/8" in length, top. The bottom row on the tape indicates that the halfway point is 14 11/16". To mark the midpoint, go back to the top row, find 14 11/16" and make your mark, bottom.