Just one little thing to clarify. I think it is best to do an experiment. Get a box of formula now. follow instruction on the box, 1 scoop for how much water? mix it and see what it comes up to on the bottle. Write it down. (it might make more sense to do at least 3 scoops, as you don't want to miss out on the round up/down.)
Then mix a jug the way you would want to do it. I think the cooling off would take considerable time, unless you use ice. (Get a feel of how long it would take, so you will not get a shock). Then after proper chilling test how much cold (from the fridge) and how much hot water give you a good enough room temp bottle. 60ml of 5C water will not need 60ml of 100C water to come up to 23C. Just the math doesn't sound right
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AND when you run out of premade one, what are you going to do?? What is the contingency? Pour hot water, add formula, stir, then add cooled water!
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That, to me, is a must know-how!
Oh and Laura, I wanted to find your other thread but I will just add here. As I had awful back pain that I couldn't even hold DS as a newborn, I sometimes fed him on a nursing cushion, the kind of microbeads, not the hard filled ones. It can nicely hold a baby in place but not too high, esp. as a newborn. You will have a hand free for twin b. You probably can prop them both next to each other and hold both bottles. Soon enough, they will be able to hold the bottles themselves.