[css-ruby] How is additional leading calculated?

https://drafts.csswg.org/css-ruby/#line-height says:

However, if the line-height specified on the ruby container is less than the distance between the top of the top ruby annotation container and the bottom of the bottom ruby annotation container, then additional leading is added on the appropriate side(s) of the ruby base container such that if a block consisted of three lines each containing ruby identical to this, none of the ruby containers would overlap.

I think this is bit fuzzy in two ways:

• "top of the top to bottom of the bottom", as well as line-height aren't just abstract distances, but positioned segments, so one can be smaller than the other and still stick out
• I am not convinced that determining what side(s) is appropriate is self-evident

(@dbaron was also concerned about that text in https://lists.w3.org/Archives/Public/www-style/2014Dec/0366.html, though the specifics of his concerns are somewhat outdated due to more recent edits clarifying a few things)

Start by reading the CSS Ruby line-height section linked in the issue, then review the cited 2014 www-style discussion and the issue's existing comments. Done means resolving how the distances and appropriate side(s) are determined, with the specification text made precise.