I'm not sure about top up etc but just to say there is a growth spurt at 6 weeks that could explain why she's feeling hungrier and sleeping less etc x
I’m currently doing this with my 4 week old. Offer breast first then top up either straight away or when he shows hunger signs again with a bottle of expressed milk or formula. Usually start with 60ml, but sometimes he only has 30ml others 90-110ml. Also due to not gaining weight and poor latch!
You can work it out based on her weight - it's 150-200ml per kg daily so you can divide that by how many feeds she has. Or just go up an oz once she's consistently finishing the bottle X