E.M.
I can absolutely relate to this!! I had similar issues with both of my BF kiddos. The first baby went to stay with a close friend who had 1 child of her own while I was in school. She was not experienced in child care and gave my son waaaay too much milk. The problem was, he often nursed for comfort when he was with me. Comfort nursing is far less efficient and they get far less milk. When they try to use the bottle for comfort, they get overfed. I was CONSTANTLY stressed about pumping enough and he did end up getting a handful of formula bottles to "make it through" until I got home.
With my second, we had found a wonderful, VERY experienced daycare provider. I gave her enough milk for 2-3 days at a time plus one extra bag with about 4-5oz. She never ran out. She fed him multiple small bottles throughout the day, to mimic breastfeeding on demand. For example- I would drop him off at 9:00am, having fed him that morning around 8:00 or 8:30. She would give him about 2oz at 10:30, 2oz at 12:00, 2oz at 1:30, etc. He never ate more than about 18-20oz in an 8 hour day, often less. Even as an older baby I think we topped out at 3 6-oz bottles and maybe 1-2 "snack" bottles, so maybe 24oz. He was 9lb 2oz at birth and consistently in the 90%, so he definitely got enough to eat :)
My emergency trick- give her a handful of breastmilk bags with only 1oz or so. If she runs out of milk she can make a bottle with 1oz milk and 2oz distilled water. That will act as a little tide-over until you get home.
Oh, and I left my little one for 2 nights when he was 10 months old with my mother. I left enough pumped milk for a week and she managed to go through ALL of it and called me while I was in Vegas for my wedding anniversary trip to tell me that my baby was "starving" 10 hours before we would get home. Sigh. They mean well, really they do.