The idea came from Terry Senate & Timmy Patterson because they had lots of shapes (and glassed boards) with fin boxes routed wrong. With so many different fin systems and routing templates it gets pretty confusing. Each one takes a different length line through the dots to align the template. The basic concept was to allow marking of the dots without any measurements (except distance from the tail along the stringer). The Fin Marker can also be used to check fin locations and toe-in on any board, or to give a marked shape a final check before routing. For glass-on’s the marking holes fit a fine-point sharpie pen.
The fin package should be symmetric to the outline because outlines are never truly mirror images from the stringer even for master shapers. 1-1/4” in from the rails was chosen because the usual 1-1/8” was really measured from the tucked edge on hard or 60/40 rails. If you are always using the same fin system all the time, you can drill a marking hole for the top dot specifically for the router template so that no line needs to be drawn.