Ask HN: Ballparking the valuation of a startup based on raised amount?

Is there a rule of thumb for estimating, as a third party, the current valuation of a startup based on knowing the round of funding they're up to and the amount of money raised to date?

Obviously this would vary a lot between different companies, but I don't have any intuition about this at all. I'm supposing that there's some upper/lower limits that you can use to estimate. For a startup that is traveling well I'm guessing that the lower limit is the amount they've raised so far and the upper limit is somewhere on the order of 10x-20x the amount they've raised so far. But that is a total guess - does anyone have any better intuition than me?

As an example, without knowing anything else about the company, what sort of ballpark would you guess for a series B company that has raised $50 million so far? Would $100 mil - $500 mil be typical?