Hi @ali_bean ,
Out of pure curiosity, I was looking for a purely mathematical way to solve this, and I have a solution that currently only uses 2 operations (after a basic trigger). Technically, by combining the formula into one field you could use no operations at all, but the formula would be massive and a nightmare to debug!
Even so, the formulas work over longer periods too without needing any loops.
This works on the basis that 2 days out of every full 7 days period will be weekend days, and you can work out how many of the remaining days will fall on a weekend using this sort of logic:
It may be helpful to someone else looking for a solution to the same problem!