[YZF]

Doubling your estimate is a common rule of thumb that goes way back but over time if you keep learning from your previous estimates you should be able to become more accurate on average. There's another subtle thing going on is that many times when the estimate increases so does the actual time, a self fulfilling prophecy, hence Hofstadter's law...

[crasshopper]

Agreed, if you're doing the same kind of work (or similar enough) over time.