You will not find the earned value formula in the PMBOK. This is becuse it's supposed to be obvious. Earned Value is equal to the completed portion of the BAC. Which is akin to what you have finished of what you estimated it would require. But the BAC is a moving target, so the formula EV = percent complete x BAC will produce different figures over the course of a project. But with each project schedule update the figure will become increasingly accurate. There isn't much complexity in the formula but it's important to understand because many other metrics use it as an input variable.
Take for example:
The BAC is 20,000. The project is half complete. This should mean that EV = 20,000 x .5 = 10,000. But by what measure is the project half complete? Unlike PV, there is no solid standard to use for that variable. It is best practice to rely on an updated project schedule and conservatively estimate your progress. If you have been meeting your milestones on time it is easier to make an accurate estimate than if you are not.
As simple as this formula is, it's critically important to derive it from as accurate a BAC as possible. However since the BAC itself is an estimate, all sums derived from it can only ever be estimates. This complicates accuracy when deriving all subsequent figures such as SV,CV, CPI, SPI, EAC, ETC, VAC, TCPI and so on.