Note that the rule of thumb is for 35mm (full frame) so, depending on your experience and steadiness, you may want to multiply by your camera's crop factor to get the equivalent rule for you, then add or subtract a stop.
Also the rule of thumb for aperture is two stops closed from wide open, but generally around f/5.6 to f/8 is best for most lenses. F/18 could start to show effects from diffraction, depending on final magnification. With such a flat subject, there's no need for a high DOF unless it was a macro shot.
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