The hand's breadth, a 4 fingered palm, may be more fundamental than the Egyptian bd (foot). The cubit was most likely a means of counting palms. The royal cubit was 7 palms but there was a more common cubit of 6 palms. The counting scheme seems to be based on the powers of two idea. 4 fingers = 1 palm. 16 fingers = 4 palms = 1 bd (foot). But the cubit doesn't fit in with this scheme. Neither 6 or 7 palms is a power of 2. There was a cubit rod found a Memphis which had the 7 palm royal cubit on one side and the 6 palm common cubit on the other. So the simplest explanation of the cubit is that it was a measuring stick which allowed the user to measure lengths without counting them one palm at a time. Lengths could just be read off in terms of palms and fingers.
The fingers of the royal cubit were marked as follows: 1finger, 2 fingers, 3 fingers, 4 fingered palm, five fingered palm, fist, and so on up to 8 fingers. The glyph for the remen is placed above the 15th finger on the scale. This appears to be what Wikipedia calls the bw (foot) which was 15 or 16 fingers. The term remen was also used to represent half the diagonal of a cubit square which was about 20 fingers. And a remen square would have a diagonal about equal to a cubit. The remen marked on the cubit seems to be associated with the forearm and not a foot. Perhaps this is an older usage that of the diagonal of a square came later.
It you look at the side of a royal cubit you will see that there are fingers which are subdivided into parts varying from 2 to 16. The Egyptian scribes needed all these divisions since they used unit fractions. But the scale shows that with 16 subdivisions they were capable of millimeter accuracy. One can compare the royal cubit with the modern engineer's scale or architect's scale.