How to get a copy of this Excel Breakpoints file?

Go to 'File' then you should have the option to create a copy, but you'll probably need a Google account. You can do what you like with the copy.

Edit: If you prefer something like Excel, then you can go to 'Download As', also found under 'File' but it may loose functionality in the conversion process.

The link you posted has "/copy" on the end. If you have a google account, the link will take you to a page with just a single button on it that says "Make a Copy". Click that button.

Then you can bookmark:
https://docs.google.com/spreadsheets/u/0/
to reach your available Google Sheets files.

just so you know, I don't think the calculations are accurate. Or the other file I have isn't.

you can also use this webpage

http://jop.space/powerUp/#!/