Breastfeeding to formula

I’m due back to work in October and I want to slowly wean my little one off the breast onto formula.

She’ll be 6months in July and starting BLW if she’s ready.

Currently she feeds 2-3am and 5.30am ish and then every 2-3 hours through the day.

I’m going to drop a midday feed first and try to keep her night feed as breast for as long as she needs or at least until 9months.

My question is how do I know how many ounces to start her formula on in the day? I don’t want to give her loads at once as I know breast milk is lighter and therefor her stomach doesn’t need heavy over filling,

Any help?