In the 1930 1931 coupes book by Marith McCoul
it states "To correctly drill the fender step hole, hook your tape measure over the outer lip of the fender and measure up over curve 8 5/8" to
8 7/8 . Chalk mark it vertically. On the underside of the fender, measure directly from runningboard mount end of the fender in a straight line up to the center of fender 31 1/2". Chalk mark it horizontally, Double check this measurement by measuring from the edge over the crown 34 1/4 and
marking that . Triangulate the hole point with the three marks, Drill.
Hope this is clear to you because I copied direct from book.