For a site I needed another breakpoint for screens larger than 1200px. I went about overriding the Drupal Bootstrap theme to add an extra large ('XL') breakpoint with the width of 1500px. This was for the SASS starter kit.
To do this I needed to override the following files by adding the extra breakpoints and changing import paths to point to the overriding files:
Please find a copy of the overriding scss files here: