Earlier quoted context omitted.
I got the number the same way journalists get any funding number: by talking to some of the many people involved in the deal. Funding terms are notoriously hard to keep secret in Silicon Valley, because everybody talks.
OK, so you didn't consult some resource online and see what the series B or whatever round was and use a formula based on what the raise was to derive the valuation? I see people throw these "valuation based on the last round" and I'm always interested at how they arrive at these numbers. Is there no rule of thumb then?
One variable is how much VC firms at a given stage typically need to own (below a certain ownership in the company, even a fantastic outcome will not cover the cost of the many failed investments that are characteristic of high-risk ventures). So that puts an upper bound on reasonable valuation for a given investment amount.
Then there is a similar variable for team dilution. Founders will only give away so much of their company before losing the feeling of ownership that is a big part of the motivation of entrepreneurs. So that puts a lower bound on reasonable valuation.
Then there is the trajectory of the company. How far are they from being IPO-ready? How many more rounds to get there? And if they don't get there, what are realistic acquisition prospects? If any similar company has been sold, what was the price?
None of this is an exact science. Sometimes people do unreasonable things, because they don't have a choice, or because they don't know any better. And many well-informed analysts also have biases or conflicts which twist their estimates.
Since you mention online resources - in my experience almost everything you read in professional ad-supported publications is biased, wrong, or blatantly one-sided.