And another reference. This one actually sounds more like what Gus was referring to. Unfortunately not a link, but enough to likely find the manual.
Gibbons' Artillerists Manual, 1860 also has some guidance on estimating distances. "On a clear day, and with ordinary sight, at from 190 to 200 yards, every part of a man's body can be seen; and although the details of dress and figure begin to grow indistinct, the grades of officers can be recognized at these distances. From 400 to 480 yards, the face can no longer be distinguished, but the head, body, arms and movements, as well as the uniforms and muskets, can. At 600 yards, the head and upper and lower parts of the body can be made out; and of the uniform, the accouterments and white pantaloons only can be seen. From 750 to 800 yards, the body appears of an elongated form. Extended arms can be seen in profile, as also the legs of men in motion. The uniform can no longer be distinguished at 900 yards, but the files can still be seen, as well as the movement of troops, and the dust thrown up by a projectile ricocheting on dry ground. From 1100 to 1200 yards, the files can scarcely be distinguished, and the troops appear like solid masses, the movement of which can still be followed."
Calum