Amount of formula to supplement at night

Hi all. First time mom and my little baby boy is 4 weeks today and I have been exclusively pumping. His appetite increased to 3 ounces per feeding and I'm unable to pump enough to keep up with his demand. I want to use formula as a supplement for night time feedings and should I still go for 3 ounces? Maybe 1 ounce of breastmilk combined with 2 ounces of formula per feeding? Please help and thank you!