Installing Addon:

1. Download the .ZIP file but do not extract it
   
2. In Blender, go to Edit -> Preferences -> Add-ons -> Install -> Locate the .ZIP file and double click it
3. Search for "Useful Calcs" in the search bar, and activate the addon by clicking the checkbox
4. The addon is now installed, no in the 3D Viewport, you can hit "n" on your keyboard, and in the panel that is shown, find the "Quick Calcs" tab