My recollection is that the original strain needed about 60% of the population immune, for herd immunity threshold, but is more like 80% for Delta.
At the time I heard that, there were caveats, that the Delta Ro was not precisely quantified. Maybe by now, it is better measured.
RO is estimated at 5 to 8.
Best anyone can do.
Will be a while before any better estimates come along- just too many other affections now..