What makes you think your milk isn't filling her up?? There are many stimulating behaviours that can seem like fussiness but it's normal especially during growth spurts.
If she's on 3 meals per day then her breast milk intake will reduce, but also they are v efficient at the breast at this point so don't take as long per nursing session. So unless there are signs like not gaining weight I wouldn't be concerned here. Of course if you want to switch to bottle feeding for any reason by all means do so, it has to work for both of you, just want to reassure you that it is probably fine :).
It normally gives a guide on the formula tin (per age and weight i think). I would start with the guide amount and see what she takes. If she finishes 3 feeds in a row increase the amount per feed