Hmmm. So I combination feed for various reasons.
Dd one was failing to thrive, jaundice etc and I ended up topping up.
Dd two I was scared of a repeat so combined fed from start. This is what I did
I started with BF but gave a tiny tiny amount of formula, like 10ml, so as not to have a problem introducing later if needed.
I appreciate they say not to introduce until BF established but I can only tell you what I did.
I then BF for first two weeks at that level fine. Then the problems started, low supply, baby not gaining, hungry constantly, so I BF then offered formula after every second feed.
Downside was I wasted a lot the first two weeks getting a pattern going. It was also complicated by silent reflux so I was exhausted.
She started to tell me what she needed after about seven days so week three. I could see that after certain feeds she would take more, others she wasn't bothered.
Now she is three bottles during day, multiple BF in day and mainly BF overnight but occasionally has had a top up if still hungry after long feed.