There’s a much simpler way that will get you within 15% in 90 seconds of less
Glen B. Alleman
1

Well, our assumptions and constraints are quite different.

I’m uncertain where 85% confidence came from though.

When I plugged this question (edit: the one in example you linked to) into my data set, and asked for 85th percentile (to mirror the 85% confidence), I got 56 days to your 59! :). That’s, of course, a coincidence.

Anecdotally, when my team attempted to estimate a work item independent of the method I described, we ended up at 90th percentile estimate. I would love to be able to compare methods (edit: if there is a mapping from a some way of estimating to a specific percentile in my method), but that’s future work for now.