That’s fine but that doesn’t mean everyone else will.
You can’t use cover on a sprite because that refers to the whole image and not just the little piece you want.
I think you are talking about how to make the one sprite as a percentage of the whole sprite?
If we take this image for example.
Then the top image percentage would be 286 / 260 and 372 / 168 which would equal background-size: 110% 2221.43% (approx).
You could use that in this demo.
However in order to stop the white gap when zooming in Firefox you would need to over-size a bit and I would do it like this:
background: url("https://i.imgur.com/tCYb8YF.jpg") no-repeat -11px -11px;
That works for me at all zoom levels.
However it would have been easier to simply reduce the element size by a couple of pixels or have the image size oversized by a couple of pixels as already mentioned.