There are definitely rules of thumb but there is a lot of margin of error. You can triangulate based on different variables. One variable is how much VC firms

There are definitely rules of thumb but there is a lot of margin of error. You can triangulate based on different variables. 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.