There is a lot of literature available which covers the ins and outs of various payback calculation methodologies to justify investing money into projects. It requires more than basic financial knowledge to understand those payback calculations. Many good projects unfortunately are killed and filed because they fail to meet the CAPEX threshold by one digit after the comma set by such methods.
The following valuing worksheet [32 KB] supports decision making based on changes in throughput (=T), operating expenses (=OE) and capital employed (=I). It is explained in detail in the book “The Theory of Constraints And its Implications for Management Accounting” by Noreen, Smith and Mackey (The North River Press 1995).
The approach of the worksheet is intuitive and easy to understand - you will be safe in your analysis or decision if your project yields a positive change in terms of EBIT and/or ROCE in a two to three year time frame (disclaimer: it is still your decision and not a direct consequence of this spreadsheet template).
Allow yourself a safety discount of roughly 20% on your calculated benefits to protect against Murphy.