You can vary a little and keep a totally usable tyre- for instance you can often go slightly narrower but with a higher aspect ratio, or wider and lower, to get a tyre the same height and therefore rolling radius, and there’s a range of tyre fitments allowable for each rim size.
Some folks say this causes insurance problems if you deviate from the OEM fitment but I’ve never found that (in fact some people say that if you fit any tyre apart from the original, that’s a “modification” but I’ve never found that either) There are tyre size calculators available online.
So, I can’t remember offhand the sizes but on my Focus, I switched to a marginally different size that ended up being about £15 less a corner, and was functionally the same. And frankly my experience suggests that you can buy tyres from 2 different manufacturers of the same claimed size, and get a bigger difference than you get between 2 sizes from the same manufacturer.