from memory - from when I last had a Kiindle
1. it did not do fractions of ems, only whole numbers
2. it could not split a line around a—emdash character, resulting in some strange looking lines. I concluded that endash with spaces either side was more practical.
( just 2 of the reasons why I now use Sony readers instead ! )
maybe later Kindle firmware is better - you could ask on kindle forum- or look for firmware upgrade release notes