What I do is to chop up the Tanganyika Strips to 80mm Sections. Then I attach 6 of them together with some clips then scribe them with a 6B Pencil.
I made this Waterline Marker Tool out of a bunch of Paint Stirring Sticks.
It allows you to Mark off the exact Mark for the Waterline along the Hull.
I used a 6B Pencil because that was the only one soft enough to draw on the Primer.