I'm going to disagree with the previous post (sorry!) and say never top up if you want to successfully mixed feed. Top ups can cause a number of problems, the main ones being:
- Babies can prefer the faster, easier flow of the bottle, if they come to expect this after a breastfeed, then they can be impatient at the breast and not have as much as they might have done otherwise, leading to a reduction in supply.
- Babies will almost always take milk from a bottle even if they're full and satisfied. Imagine after you've eaten a meal and are full, when somebody offers you dessert - unless you're really, really full, usually it's quite easy and nice to eat the dessert as well. This is what's happening but it can look like oh, they weren't really full, I must not have enough milk. So it can put that doubt in about your supply.
- When you top up after feeds it can be tempting or seem logical to supplement with extra formula during growth spurts or fussy times, which are the most crucial times to be breastfeeding as it boosts your supply for the baby's future needs.
So a combination of these things leads to a drop in supply and ultimately the end of breastfeeding, unless you're lucky. Also, it's twice the work if you're topping up at every feed.
When you want to introduce the first bottle, you need to stick to once a day at a set time, start with a smallish amount (BF babies take less milk than FF ones) and keep the FF just to that - any fussiness, growth spurts, etc, offer extra breastfeeds. Over time you can introduce more bottle feeds (once a week?) until you're up to the ratio that you're happy with. I don't think it particularly matters when you introduce the first bottle but the earlier you do, you want to give smaller amounts - no more than 60ml at 1 week, 150ml at one month, and introduce extra bottles more slowly, maybe an extra one every 2-3 weeks rather than every week.
Also pay attention to the gap between feeds. In the very early weeks (pre 6-8 weeks) you don't want a gap more than 5 hours maximum. After 6-8 weeks you're probably OK to go about 7, maybe 8 hours, once in a 24 hour period.
Until 3 months or so (when your supply changes from hormone based to demand based, you'll know because your boobs will feel floppy and "empty" between feeds rather than being engorged) I would stick to alternating breast and bottle at each feed, rather than doing two bottle feeds in a row, because that would make too big of a gap, bearing in mind you'll (hopefully!) have a gap at night as well. After this supply change has happened you should be able to shift things around a bit more easily, and easier still once your baby is established on solid foods at around 6-8 months.
Throughout the whole process stick to the golden rule: Bottles on schedule, nurse on demand. Make sure if he's asking for extra, the extra is breastfeeding. That's the way to keep your supply up to scratch and working around the bottle feeds that you're adding in.
Good luck! :)