Actually, that workbook can be downloaded and you may use Tableau Desktop to "Reverse Engineer" to determine the calculation.
In the lower right of the screen, click on the 'download' button and select the 'Tableau Workbook' option.
The workbook will be saved to your machine, usually to the default "Downloads" directory. You will also probably see it in the bottom tool bar of your browser.
Open Tableau Desktop, Choose File, Open then navigate to the directory the workbook is stored in (Download by default). Select the file to open.
Once open, navigate to the bin worksheet and find the dimension that defines the bin. In this workbook it is entitled "Results Bin".
Right click on the dimension (Results Bin) and choose 'Edit' to see how the calculation was constructed.
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Tableau Online Deployment Advisor