I see what you're getting at but I'm not convinced that the manufactures work to absoulute dead millimeter tolerances day in and day out when it comes to a fairly trivial thing such as a trap or stay wire for a beach cat. [/quote]

Excatly, so there is no point using these as the actual measuremnet menthod. Where they bi-sect the hull, YES as this is (more) constant as (having measured many boats) the trap lines are just used to provide the sides of the triangle and so need to be lengthened anyway by adding string to get to the bridal point on the hull.