How many oz?

Daisy 🌸 • Mother to 2 boys, Wife to an islander 💍🌴🤙🏼 Pregnant; Due Aug ‘19 🤰🏻
My baby is 3 weeks and 1 day today. I'm thinking of giving him 4 oz. Currently he drinks 3 1/2 oz. I saw a chart online saying if you multiply by how many lbs your baby weighs x by 2.5 meaning my baby should be eating 25 oz per day. I don't get it. Can someone explain this to me??