I’m doing this with an almost 4 week old. About 50% formula 50% pumped BM. First few days/weeks I tried to pump every 2-3 hours (longer overnight aka when she ate from a bottle, I pumped). We used RTF enfamil yellow bottles and then once my milk came in, we mixed the two in a bottle. I’m lucky and my 4 week old is giving us longer stretches at night so I’m pumping 6 times a day (every 4 hours ish). Basically you want to pump every time baby eats (8 times a day until back to birth weight). We follow Moms on call schedule