In my gutchers day a shepherds work was counted as looking after 640 yowes ie. 32 score=a hirsel, now hirsel had two meanings             {1} The work of one shepherd.                                        {2} The area of ground required to sustain these 640 yowes. This of course being variable depending on the quality of feed available in different locations.
He never mentioned any method of counting, other than the score.

Any take on where the word--hirsel--comes from, and was it unique to Scotland?.

In a recent advert for a shepherd he was to herd 2500 yowes and 60 hill cattle, quite a change in 70 years!