Combination feeding can be a slippery slope. Too much formula and your breast milk supply can diminish. Too little, when you are suffering from a milk supply already inadequate, and you can end up with a hungry, unhappy, under nourished baby.
What I have done, that has worked, is to constantly challenge my production while allowing my baby to find her own level.
This can be very tricky because babies are smarter than you think. Especially when it comes to food. They pick up on patterns remarkably fast. Growth spurts are by far the worst times for balancing combination feeding. If they know that they have to work at breast feeding, but that after some arbitrary point they get this much easier source of stuff that fills their belly, well then they’ll experiment with what they can do to to get that food easier and faster. Babies are already equipped to do this when getting milk out of the breast, experimenting with suck techniques and patterns to get the most milk the easiest way possible. This is why many mothers combination feeding using a bottle will find the baby refusing the breast, taking more from the bottle and even their supplies dwindling. This is even the case to some extent with at breast supplementers, though at least with those the element of challenging the supply is still present.
Not to say this will happen in all cases. Some mother/baby dyads are able to combination feed using bottles without too many issues.
The key I’ve found to maintaining my supply through supplementation is to set personal rules about when and how it is used. Allow your baby to challenge your breasts during growth spurts. What I did was after we had found our level, which happened to be 350-400ml (12-14oz) supplement per day, was to fall into a rough routine. At certain times of day supplement was given, but between those times I allowed as much breast access as possible and always tried to initiate breast feeding before any supplement was given. Growth spurt times did involve a lot of cluster feeding, but rarely increased supplement intake. If increased supplement intake (above the arbitrary threshold of 400ml) did occur it was only for a day at most. Sometimes I did need to give the supplement earlier, but sometimes I was able to give it later as well. Constantly challenge the pattern by offering breast instead. I did sometimes split the supplement into smaller amounts to stretch us through the day when feed frequency increased. It’s likely this mimics a normal breastfeeding experience. And if you need to deviate from your routine, don’t worry about it. One day won’t hurt too much. Just get back on track when you can.
In my case after a supplementary feed it was like a clock was started. After an hour my baby was likely to want some breast again, but only a snack, often to aid a nap. Then in increasing intervals until the next supplementary feeding. If we fit a nap in there we delayed some, but the pattern still held. When I tried to delay or drop supplements I found her feeding frequency increasing with duration decreasing until it degenerated into agitation and refusal to latch. I also closely monitored her weight gain to ensure she was gaining weight. I had several upsets early on where I tried to decrease the supplement amount and she experienced weight loss. This was part of us finding our level as to how much supplement was necessary for continued appropriate weight gain.
I should say there are other ways of achieving this result, but they often involve pumping. I wasn’t able to manage pumping after feeds or pumping in place of supplementary bottle feeds, both due to sanity and my babies preference. This is one of the main reasons I moved from bottles to an at breast supplementer. I hoped that the at breast supplementation would combine the necessity of giving supplement with the breast challenge of pumping after feeds.
The bottom line is you have to be really rigorous about preserving breastfeeding if maintaining your supply is important to you.