When I went to measure out my fin placement I found one side of the shaped board if 4mm wider measured from the stringer. When placing the fins, I measured 1 1/4" from the sides 7" up from the tail (measured twice and used a square). When I measured from the stringer it was 144mm one side and 140 the other. Should I just leave it or make the fins equidistant from the stringer, ie 140 and 140?
Check the stringer for straightness: it might very well be that it is not straight. If you used it as the axle of symetry when drawing your template, you will be “wider” on one side. Stretch a thread between stringer at the nose and stringer at the tail and see if there is a noticeable difference.
Oh, and: set your fins in relation to the rails, not the stringer.