I combine FF and BF for different reasons.
1. I enjoy the break
2. I am not an expresser - would rather be spending time with my 3 year old when my bubs is down for sleep, plsu I find expressing makes my nipples sore.
3. I am returning to photography sessions and will need to be away for the mid morning and mid afternoon feeds
4. I like having some freedom during the day

So, I introduced 2 FF's a day - it works out to be every 2nd feed after the first big morning feed of the day - so around 10-11am and 2-3pm. I just make up the 150mls recommended on the tin and go from there. Sometimes he will drink it all, other times he will only take 80 - 100mls.

I breastfeed all top ups and every other feed - and because I get that break I enjoy BFing him a whole lot more.

I didn't introduce with with my 1st child until around the 3 month mark (Koby is 7 weeks).

By the time I introduced it with Paige it meant her day feeds were formula and her morning and night were BF's.

I know this is different as I am not topping him up with formula, but I thought it might help others.

I always introduce formula under the condition that emotionally I am ok if they wean from the breast altogether.