Breastfeeding Question

Hello. FTM of a 3 wk old. I'm exclusively breastfeeding my lil one. I plan on going back to work part time when she is 12 weeks. I am considering starting to use formula for a couple feedings. Does anyone have any advice to share? What are your thoughts on adding formula in addition to breastfeeding? If you do supplement with formula what feedings do you use formula vs breast milk? Thanks in advance for any advice/tips.