Great find, many thanks ! Seems officers could disagree about the space a man would occupy in the ranks.

The 1791 regulation (école du soldat, no. 221) says that during the marche par le flanc (all men stand aligned in the ranks, make a 90° turn, and then march forward), the foot of the man is replaced by the foot of the man behind him:

https://books.google.de/books?id=pxdLAAAAcAAJ&pg=PA67&dq="chaque+pas"

In no. 219 it is said that they march in pas ordinaire.

In no. 23 the pas ordinaire is defined as having the length of two pieds.

Because nothing says that in the marche par le flanc on the command marche! the distances shall be extended, the men must have been occupying two pieds of space already when they were standing side by side in the rank, before they made the 90° turn.

An old officer in 1829 shared my interpretation:

https://books.google.de/books?id=zRh3CzWwiq0C&pg=PA1&dq="deux+pieds+dans+le+rang"

Interesting though that in 1813/14 at least some officers assumed a smaller frontage. I found your reference also in Bardin's manuel of 1813:

https://books.google.de/books?id=z_5KAAAAcAAJ&pg=PA324&dq="rangs+composé"

Maybe the constant decline in the average height of men (and thus also the width of their shoulders) between the revolution and the end of the Empire, led to a re-evaluation of the space needed.

It is an abstract mathematical approach anyway, good for quick calculations. Nobody will have measured the width actually occupied by the men.