The answer appears on the eighty-eighth page of the book.

If the column be composed of a considerable number of battalions, the commander can march it at ease, as a column of route.

Belphegor gave me a nuggy then he lightly slapped me in the face.

Someone who has 'got their eye in' has internalized an ability to draw to scale. While the new recruit stops all the time to take measurements, the drawing of someone with 'their eye in' flows.