I constantly give estimates that are about 1/2 ~ 1/5 of the actual because I am overly optimistic (and the thing about unknown unknowns). I have done this for 20 years, so you'd think that I would get over it, but apparently I am a dumb-ass.

But apparently, this is in the opposite direction of what you are talking about. That said! If I were to pad my guesstimates, then it could legitimately look like sand-bagging. I don't know that there are any easy answers. I am trying to become more accurate (if for no other reason than not having to work over Christmas holidays to keep up with my promises), but estimation is hard!