When we switched to formula I mixed it with EBM for the first few weeks then went onto one pure formula bottle, then two etc.. So we really slowly introduced it. We had spit ups still, but he was growing and happy, so I didn't worry too much. When I made those first bottles I would use like 4 oz of breast milk and 2 oz of formula, don't count the breast milk as water for the formula because it will constipate. Good luck, hope this helps.