GMorrison
Its wider than you think, maybe 4 feet or so
Somewhere in my pile of refernce material is a guidline to boot topping widths, which was by class/size of ships.
And, IIRC, the widths are a guideline, as it mentions that ships at sea benefit from wider boot topping than ships largely in harbors in peacetime.
I want to say that battleships were recommended a 6' boot topping, as that allowed a lot of steaming (and using up expendibles) whitout showing any read bottom paint.
During wartime the boot topping width was doubled, as ships were more likely to expend ammo as well as ordinary expendibles; also to cruise to much more extended range.
The wider boot topping was also considered to help confuse exactly where i nthe water the ship was, which would confuse the range estimate.
The captain of the ship was the final arbiter of just where the boot topping started and stopped viz-a-viz load waterline. Some captains would go 1/3 and 2/3; some 1/2 & 1/2. Often, this was set out during dry docking when a ship would be at its lightest. Weld lines would be scribed on the hull to guide painting layout (the hull numbers & name were similarly scribed).
6' at 1/350 is 0.205" (5.22mm)