I’ve always had a full in milk supply but now but is only breastfeeding first thing in the morning, before lunch, before dinner and before bed.

I’m only able to express 40-60ml at a missed feeding should I be topping buns up with formula if she’s only getting this much milk now? I know expressing isn’t a good indication while breastfeeding but if she has a breastmilk bottle instead of a feed she’ll happily finish 150ml and I’ll still only express 40-60ml.