@Cantfindthewordsddstruggling there is a helpful Facebook page I'm on called ATDTfb (it's from the FEAST group 'around the dinner table').
I've learned a lot of helpful info from that page, and specifically about 'state not weight'. To a huge extent bmi and wfh are unhelpful for people who have ed. As you say they are based on population statistics and not on individual information, and their creation historically often have non scientific other (eg political) aims. I think you're right that 100 wfh is weight and height falling at exactly the 50th percentile for a person of that age, and so doesn't account for anyone who is a different height or weight.
If your daughter is refusing to gain more weight, that's probably an ed behaviour. I've cut and pasted a response from someone on that Facebook page below about what 'state not weight' looks like:
^*WFH is generic and thus meaningless-so bizarre to assert that all people of a certain height and age should weigh the same. No wonder outcomes are not great.
State not weight and weight influences state.
We did have growth charts and my daughter needed a full 10% above where she “should” be.
What I say recovery target is:
Able to eat all foods freely and above minimums (different for ARFID)
A healthy relationship with movement that includes rest and other stress relievers than exercise
No behaviors and significantly reduced thoughts that are not acted upon
Body image issues no longer pathological (return at least to socially normed body discontent which can benefit from therapy)
Developmentally appropriate insight into their eating disorder and need to engage in relapse prevention long term*^