Help be my brain!

DS is 9 months, drinks expressed breastmilk at daycare, 4 ounces every 2 hours, total of 16 ounces. As I wean from the pump, I want to start him on formula (have somewhat of a stash), but am confused about amount of formula to send.

I would like to attempt to give him a straight up formula bottle, no mixing with breastmilk. BUT I know that babies who have formula are drinking much bigger bottles much less frequently (in general). So if I want to replace one of his 4 oz breastmilk bottles, where do I start? Send a 5 ounce formula bottle? Would love to hear your experiences.

I don't think this should be this hard, but I'm pandemic'd out and in busy season at work. Can't think! Thank you!