That in one thing that needs to be done by the seat of the pants.I saw one done years ago,the guy had copied a fender on another car.If the arches and crowns are pressed in just a fraction different,the position of the hole won't be the same.The one I saw was measured,and marked to a T.But,when he drilled it and mounted the step pad,it was tlited to the outside and rear of the fender. It wasn't a big deal,he just repositioned it until it was flat,and drilled another hole.It was about 3/4 inch from where it needed to be.The pad covered the extra hole anyway.Lay it flat,get it level,mark and drill it.