It looks like the long arms and vintage domestic machines use "stitches per inch" whereas many of the domestic machines use "mm stitch length". There is a table called
Stitch Length - Conversion Table at
https://blog.treasurie.com/stitch-length/
there are some other conversion tables and charts at
https://aimforquality.files.wordpres...hs-3.png?w=529
https://www.generations-quilt-patter...sion-chart.png
https://www.generations-quilt-patter...sion-chart.png
https://help.brother-usa.com/app/ans...ngth-settings#