I'm a bit worried by your health visitor's advice - cow's milk isn't an adequate substitute for formula / breastmilk for an 8 month old, so it would be hard to say how much is "enough".
At the same time I appreciate that you can't force formula into them, and if you want to stop BFing then that's entirely up to you. If it was me, I would keep trying with formula, do more smaller feeds if that's what your DD seems to be happier with even if it is a faff to keep making up small bottles. It's supposed to be 500-600ml per day, i.e. at least 16oz, if you stop bfing. I know you probably know this, and must be really frustrated that she won't drink more, but I'd continue to aim for 16oz a day at this stage, or obviously a bit less while you're still bfing.
Of course, if she just won't take that much then you'll want to make sure she has plenty of calcium rich foods in her diet which can include dairy, but also tinned fish is very high if you mash up the bones and give her those too (no more than 2 portions a week of oily fish for girls though). Some dried fruit are good, apricots for example I think. Anyway, you can google it.
Also, if she's having less than 500ml of formula a day then make sure she has vitamin drops - this is recommended anyway for breastfed babies from 6 months (formula is fortified, so ff babies get their supplement that way), but vitamin D will help her absorb calcium.
HTH