Thank you for the answers.
calibre-with-kfx is running "calibre-debug -r "KFX Output" -- <infile> [<outfile>] [options]" - is that what you mean by "KFX Output CLI"?
The part where Kindle Previewer does its magic I investigated pretty
thoroughly before switching my focus to what Calibre does (or does not do)
Just trying to understand the behavior - I do not think KFX Output converts those units and I am quite certain that KPF/KDF output from Kindle Previewer contains "em" units in some cases. However when I got resulting KFX from Calibre GUI conversion - all units are "%"...