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Another way (and more accurate) is with point source (lamp in the cardboard box, with small hole on one side). For this you need a free horizontal surface (floor) more than twice the estimated FL.
Improvise the holder for the mirror in vertical position (but be careful not to let it fall on the floor, of course). Position the point course away from the mirror and try to focus the image on the same surface near where the pinhole is.
The distance between a hole (image) and the bottom of the mirror will be exactly twice the focal length .
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