I pump then supplement with formula...have been since almost day 1. I will keep the BM separate, but with the formula part I have been putting the bottle back in the fridge if DD hasn't finished it all. Then at the next feeding I will warm the bottle under stream of hot water and add some mixed formula to that then feed.
I just read online to never give a past bottle to LO if they haven't finished it due to micro-organisms....I feel so guilty!! 
                 
                BFP: 12/3/15     EDD: 8/11/16     IT'S A BOY!!!
MAXIMUS POWERS   8♥5♥16
BFP: 8/5/13      EDD: 4/13/14     IT'S A GIRL!!!      
AYLA BLAIR   3♥27♥14
Re: oh no..just realized I am doing something wrong!
MAXIMUS POWERS 8♥5♥16
https://kellymom.com/bf/pumpingmoms/milkstorage/reusing-expressedmilk/