When to up an ounce of formula

This might sound stupid but when do you know when to up an ounce for formula feeding? My baby’s currently 10 weeks old, was born at 8 pound 13 and when last weighed over a week ago he was 12 pound 10.

He is currently on 5oz and goes 3 hours between feeds, sometimes longer. I feed him around 10pm and he goes down until 3/4. He has always been a good eater and finishes his bottles but also sometimes he can leave an ounce or two in the bottle too.

I was stood starring at the back of the formula box tonight when I noticed it said 12oz - 6 ounces. Should I be upping it to 6 and when do you know to just up it? I can’t even remember how I knew to up from 4 - 5 (newborn blurr) xx