There is no such thing as a correct value.
There are different ways to calculate (read: guess) an anticipated value, but neither of them is the "correct" one. Last not least this depends on your interpretation of the term "correct" in that context.
- Book value / value of oustanding shares
- Discounted (future) cash flow
- Value (which one?) of tangible and intangible assets
- Anticipated future market share
- and so on ...
Why do you think paid Facebook such a huge amount for WhatsApppaid Facebook such a huge amount for WhatsApp? Surely not, because it was the "correct" value.