Depends on who I'm talking to. An estimate is not actually a number, it's a probability distribution. Few people want to hear that; they want one number to summarize it. But not everyone wants the same number.

My team wants to know the mean; my manager needs to know where the cumulative distribution reaches 90%; and marketing needs to know where the cumulative distribution reaches 99%.

In my experience, a developer's educated guess hits very close to the median completion time. Software task completion tends to be lognormally distributed, so to get the appropriate scaling factors, multiply that educated guess by 1.6, 5, or 10 respectively.

One of my university professors advised "multiply your best-case-scenario estimate by Pi". It's still absolutely plucking numbers out of the air, but I've found it to be pretty reasonable over the last decade or so...

Using pi instead of 3 has the obvious advantage that your estimates are much more precise.

Absolutely. I find it far more accurate and when it's not, it's better to underpromise and overdeliver than to overpromise and underdeliver.

I use 4 - double and double again.

But yes, I subscribe to the approach of make your best guess, then multiply by 4.