Is she having a dreamfeed? Tracy expected LOs to have a DF until 7-8mo too. If there's no DF and that one feed is the only one that's really good going and it's doubtful she doesn't need it however much she gets in the day. Even with a DF, one night feed is still completely normal.
Tbh, breastfeeding research has moved on significantly since the books were written and we like to think Tracy would have updated her BF advice accordingly had she still been around.
One way to help increase calories is to do breast compressions whilst feeding to help dislodge the fatty molecules from the milk ducts. Are you letting her finish at one breast and then offering the second side?