This is partly a bottle feeding issue, not just a Formula issue, isn't it -
I can see very big differences in the way that DP/MiL fed DS expressed milk from a bottle, and the way that DS fed from the boob. MiL in particular, whose other grandchildren have been FF from the start, got very worried if DS didn't gulp down a lot of the bottle straightaway, because she doesn't like to leave the bottle standing around. While broadly supportive of BF-ing (mostly...), she has a deeply grained obsession with timed feeds, and wanting DS to be 'properly hungry' before a feed. This translated into a strong anxiety about the amount DS took from a bottle.
She clearly saw it as a 'failure' if DS didn't take a lot in one go, which I think led to some stressy times for both of them when she was left with him. She kept nudging him to take more when he wasn't interested, and also seems to have ignored some cues when he did want to feed ('he can't be hungry yet, I thought'), and when I got back to them, she'd behave as if she was breaking bad news if he'd only taken a small amount. I did try and reassure her, and said if he didn't eat much then, it was probably because he wasn't hungry! and he'd make up for it later, not a problem... but it didn't stop the maths anxiety.
DP was very aware that pumping and storing milk was a bit of an effort for me, and that a lot of milk poured down the sink meant I'd get supply issues because of over pumping... so although he tried to follow DS's cues when offering EBM in a bottle, I could still see him worrying about how much to offer. He seemed either to be angsting over wasting several ounces, or be racing to the fridge to get more after DS speedily polished off a micro-feed.
As tiktok says, it's virtually impossible not to hold the bottle up and see what's left, and do sums. When feeding DS from the boob, I have never ever had any idea how much he's getting, I've simply fed on demand, and still do at 9 months. His weight gain has been textbook so he's obviously done fine, and tbh I think that relaxed attitude has helped us hugely with weaning, which has been a very happy and chilled process so far.