If your boat is on a trailer with some space around it on one side at least, and you have some spare rope laying around, you can do this empirically and easily. Run a rope from the pylon straight back maybe 40 or so feet. Doesn’t matter. Anchor it somehow… stake, weight, whatever. That’s centerline. Then run a second rope parallel to the CL rope 37 1/2’ away. That’s your buoy line. Then attach your ski rope at whatever length you are interested in checking, add whatever you want for your arm/body extension, swing it out to the buoy line for each different length, and mark the gunwale with tape where the ski rope is for that length. Only takes 15 minutes maybe. One thing to note is that the ski rope is not laser straight from the pylon to the skier when the skier is at apex. There will always be a bow in the rope and the skier will likely be higher on the boat and wider on the course than indicated by gunwale marks. That just means if you make your mark you’ve got a little width to spare. (That’s why you can’t zoom in too tight with camera tracking devices that use the ski rope to track the skier. The skier will ski out of frame at apex if zoomed too tight because the skier is ahead of the rope at the pylon so the camera points behind the skier, not directly at him.)