Advice please

Lacey

Switching 7 week old baby to formula.

Currently doing 2oz breastmilk 1oz formula. She drinks it all then an 1.5hr later she's hungry again, drinks another 2oz of the combination. Tried uping it to a total of 4oz but she won't drink it all at once. Is it because the breastmilk digests faster? Should I just go right into all formula? I didn't want to shock her tummy, I plan to do 2oz formula 1oz breastmilk for tommorow's bottles.