The tick marks are based on the width of the planks at a particular bulkhead and enable you to determine the taper of the planks. Typically the most full size planks would be found in the middle of the ship.  ( amidship )   Imagine this is your middle bulkhead.  I have marked the 5mm width of the planks ( not scaled ) and determined there are 20, with the bottom plank ( garboard ) being a little wider. That will have to be accounted for later. At the bow of the shi