Well, when I had to supplement with formula for my DD at daycare, here is how we did it. I would take whatever I pumped in a day (say, 12 oz.), and divide it evenly into 3 bottles (4 oz. per bottle). I would send these 3 bottles of BM to daycare, along with 3 empty bottles that daycare could fill with formula. (Daycare provided formula... We just used regular Similac, as DD took it just fine.) I would have daycare give my DD her 4 oz. bottle of BM FIRST, and if she sucked it down and still seemed hungry, they would then give her an ounce or two of formula in a second bottle. If she didn't finish the 4oz of BM, then they didn't worry about offering her any formula. If daycare offered her her 4oz of BM, and she only drank 2oz, they would wait 30 minutes or so, and re-offer the BM again, to see if she was still hungry. They knew how much I hated to have my milk wasted, so they would offer it to her a couple times, but not PUSH it on her, or force her to drink it. Once a bottle of BM is warmed at daycare, they only have 1 hour for the child to drink it, or it has to be thrown away, so the caregivers would offer it to my DD several times during that 1 hour period before giving up and throwing it away.
We used this same pattern for each of her 3 feedings at daycare, and she was at daycare for about 9 hours a day, when she was taking bottles. This method makes for a LOT of bottle washing (6 bottles per day, if daycare used them all), but allowed us to minimize the amount of formula she was drinking, by having her drink ALL of the BM first, before offering any formula at a feeding.
I hope this makes sense... HTH