[iamasuperuser]

Software estimates are unavoidable. I find this approach the best; rapidlyeatimate.com It works well to bypass anchoring and biasing by initially focusing on ‘divergent thinking’. It's as accurate as any I'm come across.

[kqr]

That seems to make the mistake of summing up the lower bounds to create a lower bound for the total time (and correspondingly sum the upper bounds to create an upper bound for the total time.)

That yields a very unrealistic (as in not-relevant-for-reality) range for the total time. Am I misunderstanding something?