How does Boss HP scaling in flexible raids work?

Solution 1:

Perhaps this is better served as a comment since I haven't found anything authoritative, but the closest information I've uncovered is:

1. Anecdotally, from this thread, some players report that each new person adds ~4M HP to the boss. (Brackenspore, in this case; it may vary by boss.)

2. This page has a copy of a blue post (and a link to the original) indicating that boss abilities (not HP) are probabilistic added based on the number of raid members. It also includes the information that boss HP scales more slowly than player damage, fwiw. (Do note that this post is 1+ year old and the information may have changed.)