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The question:
How can I find the quarter full mark using a dipstick in a [horizontal
axis] cylindrical fuel tank as fitted to a truck?
The answer:
Cut a circle from card.
Draw a diameter, and a perpendicular radius.
Cut a semicircle out on the diameter line.
Balance the semicircle on the radius line with a pencil tip for the pivot.
This is the centroid of the shape, and represents the center of mass of
a similar cylinder - this is about 40% of the radial distance from the
center.
A stick marked with three lines scaled to the diameter of the fuel tank
in question
therefore fits the purpose. A mark at 70% of the diametrical length,
one at 50% of the diameter, and one at 30% of the diameter.