You could use a worksheet change event. This would however rely on users enabling macros when the workbook is opened.
Using the calculations in your example and with column C formatted as a percentage:
In case you are unfamiliar with VBA. Hit Alt + F11 to open the the Visual Basic Editor (or select Tools - Macros - Visual Basic Editor).
Copy and paste the code into the relevant sheet in the VBA Project.
To lock the VBA Project: Right click on VBAProject and select VBAProject Properties. On the Protection tab tick Lock project for viewing and type a password. Save and close the workbook.
I have attached a workbook (without the VBA Project protected) as an example.
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