In the lead-type days, an em was the width of a capital letter M. That's no longer true, but it's good enough to go by. My style sheet uses ems to designate paragraph indents (1.5 em), blocks of text (1 em), and margin-top and margin-bottom (usually 0.0 em but that can vary).
I use percents to designate font size (200% for the paragraph style for the book title, 150% for chapter heads and the paragraph style for the subtitle, 125% for sub-heads and the paragraph style for author and publisher).
https://notjohnkdp.blogspot.com/2013...yle-sheet.html