I thought I'd have a read through here to see the other side of things as 30 years ago I put my parents through this. It's interesting and you are all loving parents doing your best with the odds stacked against you. I thought I'd add some things that might help with long term recovery.
Many will disagree, and be vocal about it, so I'll say now I will leave the thread immediately after posting to avoid getting dragged into a debate. Long and short term strategies are different, and if you are still at the early stage the process the short term strategies many be the only way. The early stage is the hardest, the most emotional, you do what you have to.
Firstly Anorexia is rarely about body image or size or even weight, it is often about control. It is common among high achieving perfectionists. It can be triggered by a feeling that life is spiralling out of control. It can be an exam issue, a friendship issue, a relationship issue, very often a school issue (bullying, lack of control over the school they go to or the exam subjects they are required to take or even control over what extra curricular activities they are allowed to do). It might help to sit back at consider what freedoms your dc has, and how controlled their life is.
With that in mind can you see how difficult it is for your dc to accept you taking over their food intake. Already struggling with the lack of control in their life, and having chosen food as the one, possibly only, thing they can control this is taken away from them. It exacerbates everything and feeds the ED. Obviously you have to take control, but this may explain the extreme reaction you get.
Next, once through the initial stage of getting your dc to a point where their life is not in danger please consider the following points:
Recovery is a long term goal. In the same way crash diets don't work long term neither does stuffing your dc with butter and cream at every opportunity. Please don't do this, for many reasons. Firstly it is extremely hard on the digestion. You know that feeling you get after a a few days in an all inclusive hotel eating very rich meals? Or after xmas? You feel bloated and crave beans on toast. Well, you are expecting your dc to eat like this day in and day out, and then act depressed when they push back and refuse a meal. Of course they do, their bodies are rebelling, they feel uncomfortable, they crave something refreshing and light the same way you would.
There is a reason hospitals say 3 meals and 3 snacks. That's normal meals, not overly rich heavy ones. You need to re-educate normal eating. You teach your dcs what a normal meal is, so that when they are at target they can gradually remove the extra snacks but retain the meals and retain the weight. Yes weight gain is slower but crucially it stays on. The risk with the loading with butter and cream approach is that when they move on they have no idea how to feed themselves to retain. They of course will not load with fats meaning what they think is ok, isn't ok and all their work will be lost.
Learning to eat normally is the ONLY way to avoid relapse.
Also, I note the comments about no school meaning no school work. OK, we have all just lived through lock down. It is well documented how teenage mental health was severely negatively impacted by lock down. Staying at home, deprived of school, or sport, of social clubs.... that's lock down. It is also the preferred treatment for anoretics. So, we are taking a teen with a severe mental health problem and putting them in a situation proven to worsen mental health. Anyone see the issue here. It's a disaster waiting to happen. They are stuck at home with literally nothing to do all day but obsess about their next meal.
FGS if your teen is bright, driven, musical, artistic, whatever it is allow them an outlet. You may need to limit is but give them something so they can keep in touch with their non ED self.
Now to wfh. Stop obsessing, your teen is an individual not a statistic. Look at them honestly. What is their natural build. From what I understand 100% is bang in the middle of the normal range. Well the normal range is a bell curve. They may naturally lie anywhere within that bell curve. Don't force their body to be something is shouldn't. Some small framed girls will be perfectly healthy well below, others may need to be above. Be honest with yourself, where on the bell curve should they be? Forget numbers, look at their skin, hair, energy, periods. If normal, they are fine.
Finally to the PP whose teen is working with a PT, well done. PTs have a different mind set. A healthy one. They are about building healthy, strong bodies. They educate about nutrition. They are fabulous, inspiring people to be strong. It changes the whole attitude to weight gain. No one, especially not an anorexic in recovery wants flabby thighs a saggy butt, and weak torso. Sadly, without some form of weights work thats what fast weight gain will give you. PTs also work on the brain and help teens decide to recover. As you well know, there is no recovery until the patient decides they want it.
I wish you all luck, and your teens even more luck. 