Supplementing formula with breastfeeding

My LO is 11 weeks old and has been exclusively breastfeeding. She’s never had formula before but I’d like to implement only one bottle of formula a day and the rest breastmilk. I go back to work next week and I think it’ll take some stress off me to not have to pump AS much and just kind of train my boobs to make one less bottle of milk ( if that even makes sense). My question is, how do you supplement the formula, what time a day would be best for her to have this one bottle of formula and will it mess her stomach up at first? The dr. Gave us nutramegin although she does not have any allergies.. will this make her develop a cows milk allergy because as of right now she’s perfectly fine with my breastmilk and I eat and drink dairy every day.