According to Microsoft;
“this calculator can be used to provide a loose estimate. Once you download the calculator, you will see that there is an opportunity to adjust some of the settings on the Input tab such as the user profiles, working hours, and time zones for your organization. The numbers included are what Microsoft has observed in the field. You can also adjust the number of clients per site and what profile each of those users have on the Client Mix tab.”
If you want to get more understanding of how the calculations are made, there are 3 hidden sheets you need to refer to.