> I think I can work out a 95 % estimation I am willing to stake money on even without the things you mention.
I doubt it. It's dicey to do estimations even when everything is well understood, but if you lack detailed and accurate records of past work for very similar projects... the best you can do is pad like crazy, cross your fingers, meet the deadline and say your confidence interval calculation was "accurate."
If the lognormal distribution is correct, the 95% confidence interval is a bit over 5 times the developer's gut-feeling estimate. The 99% confidence interval is 10 times the developer's gut-feel estimate. We found a lot of data to back this up, but your mileage may vary.
I doubt it. It's dicey to do estimations even when everything is well understood, but if you lack detailed and accurate records of past work for very similar projects... the best you can do is pad like crazy, cross your fingers, meet the deadline and say your confidence interval calculation was "accurate."