Some Principles of Frontier Mountain Warfare by Wilkinson Dent Bird

In deciding the strength of a column for an expedition against the North West Frontier tribesmen, the first requisite, having regard to the foregoing considerations, is so to limit numbers, that, in the topographical conditions likely to be met, the force can, as an entirety, make a march of reasonable length, let us say eight or ten miles. Assuming that the column will move at an average rate of about two miles per hour, that, in order to avoid risk of disaster, it is, as a rule, desirable for the main force not to march before dawn, and to be in camp by dusk, and that ten -- twelve hours of daylight are available, it is clear that the road space occupied must not, for a ten mile march, exceed ten to fourteen miles. That is to say, the numbers that can fulfil this condition on a narrow track, amount to about 4500 fighting men, carrying three days' rations on the person, and five days' on transport animals.

Suppose that four such columns are destined to invade the Afridi Tirah.