I have also been exclusively breastfeeding and my LO just started IFC yesterday. I provided them 120ml. Good thing about my IFC is they inform me how much my LO drinks. Yesterday was 100ml and 80ml Today is 70ml So probably i will just prepare 110ml from tomorrow onwards so not so much goes to waste. They did mention that for full day IFC, normally 3 portions is enough. But they will ask to provide 4 just in case. So if your LO needs more, they can just top up
You can follow the chart stated subsequently adjust the ml/feeding gaps according to the needs of your LO. Once you are in the centre, you may wish to inform them how your LO are being fed. This is what I did for my LO that just got to IFC.