If you're just trying to convert millimeters to Stitches per Inch, I use this chart:
https://tv-sewingcenter.com/sewing/m...tches-per-inch
If you're thinking of determining the length of each quilting line based on even stitches, I wouldn't recommend that unless you are only stitching identically sized quilting lines throughout your entire quilt. It will not look right to have many different lengths of stitches throughout your quilting because you wanted to have the ones for each stitching line even. Even the top award-winning quilters, from everything I've ever read or seen from them, just take a smaller stitch to get to the end of a stitch line. You don't start/stop quilting lines that often & the dozen or so times you take a smaller stitch will be much less noticeable than having three dozen different stitch lengths throughout your quilt.
2.75mm is a good length for quilting stitches. That works out to be about 9 stitches per inch. You could also go a bit larger at 3mm or roughly 8 stitches per inch.