ep = (ROIC − WACC) * C
The cost of capital is an opportunity cost: it is how much you could have gained with your capital by investing it elsewhere. For reasons I do not yet understand, this WACC is different for different companies, depends on how cheap the company can get credit, and is complicated to calculate.
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Economic profit is not the whole story. What you really want is the projected economic profit for the whole future. You can do this by assigning an expected ep for each year to come, and then discount them with a certain percentage. The value of your company is then the capital invested in it plus the discounted economic profit for the future. This is identical with what is called discounted cash flows (DCF), a method that estimates the cash flows to and from your company for each year in the future and discounts them. The discounting percentage is again connected to your WACC, and contains also the risk for guessing wrong. It can be calculated.
When your company is publicly traded, this still is not the whole story. The total shareholder return (TSR) depends not only on the value the company creates, but also on what people expect it to create. If it falls short of expectations, then stocks will fall and people holding them will lose money, even though the company was successful.
The book I read uses the example of a hamster wheel. When it runs slow, it is easier to surprise people by accelerating it, and create TSR. But then it runs fast, people are now expecting better performance. Eventually, you cannot speed up any more, or keep up with the speed, and stocks go down. Expectations are often irrational, so your influence on TSR is limited at best.
Read some interesting blog (named Limited Pie) by a stock market trader. Lots of good links there,
like the Internet Archive, or a page that generates random numbers.