She's eating, which is great. How much does she decide what she's eating? If you decide within her limits (e.g. she says '1 piece of fruit', if you swap out the peach and replace it with a banana, you've almost doubled the calories), then you can fiddle with the calorie content.
There is no way DD1 would have half a portion of porridge, a banana, 5 raspberries, 2 strawberries, a handful of blueberries and a glug of double cream for breakfast, but by blitzing it in the nutribullet, all she saw was 'a glass of smoothie', but it was 522 calories.
Wherever the starting point is, there will be a way of notching it forwards. But at some point you have to say 'Enough, you're not eating in a way that sustains you, so I have to take over'. It does help if a professional can start that off, though, so that you're just the custodian of the plan. I tell my DD that I'm in a team with the hospital, so we agree on the plan. Then I just get out the plan.
It is relentless. Now though, she knows I'll say 'you eat, you drink Fortijuice, or you go to hospital...' They are her three choices. Genuine choices. If she doesn't want to eat, I'm happy to measure the correct quantity of Fortijuice. If she won't do that, I'll phone the hospital.