hemp sling construction

[jvrjenivs]

First, I've to say, thats a very nice sling construction!

And I assumed that as a general rule the sling should be long enough for the cup to fit in the arm pit.

As for the length, you seems to be right for what I would call a 'medium' sling. I've to check the sources again to point in the right direction.
But I would say there are three types of slings (when we only take the 'all rope sling' as shown here into account).
A short, a medium and a long version. It's just the same as with modern guns/riffles, they are for different jobs. For short distance with high precision a short slings is much preferred, where for long distance (with less precision) a long version does a better job.