If I’m making a bottle without using my prep machine, depending on the size of bottle I’m making, but for example if I’m making a 5oz bottle I find 2oz boiled water and 3oz cooled boiled water makes it the perfect temperature. I personally store my cooled boiled water in a spare sterile bottle. Tommee tippee have lids for their bottles that aren’t teats so it is handy! If I’m making a bottle out of the house I bring a thermos flask of boiling water and again the same sterile bottle of cooled boiled and use the same method! You will find what works for you :)
Your measurements are roughly right as are the above comments. Have a few goes at home to get the right temp for you little one. If you’re not going out for too long then I do a completely hot water bottle (with the formula in it) and take it out, it takes around an hour to cool to drinking temp and then that’ll keep him happy for 2ish hours x
U can put mam bottles in water to warm them up I've been doing it since my baby was born, premaking the bottles and then putting them in the bottle warmer, u just have to unscrew the top of the bottle a bit to allow a bit of air in otherwise it does create a vacuum and suck the water in through the holes in the bottom but if u unscrew it it's absolutely fine this is what they advise to do on the mam website if u want to warm up milk in bottles. Premaking them and warming them up is much safer then the hot shot method studies have shown this. If u do hot shot method the hot bit of water u use to sterilise the formula cools down to quickly to actually sterilise the formula and 1oz of water can only sterilise 1 scoop of formula so if u put 6 scoops into 1oz of hot water it doesn't get sterilised this is why the NHS advises against it and this is why the nuby doesnt advise that u use it this way x
Feeding three, I’ve learnt the best way for us, really quick and safe! (Approved by our health visitor). So we boil approx 1.5 litres, in a formula pitcher and store it into the fridge so it’s cold. (We do this after the bedtime bottle so it’s ready for the next day) come bottle time, boil the kettle as normal, 50% boiling water, add powder and mix, then top up 50% with the cold water from the fridge. You may have to adjust ratios depending on what temperature baby prefers. 50/50 makes it nice and warm which all three of mine love :)
ah its tricky to say without a rapid cool flask, can understand not wanting to spend the money though - formula feeding does come with alot of extra stuff and it does get expensive. but i did get my rapid cool cheap secondhand on facebook marketplace, might be worth having a look? so i dont know if its right or wrong but this works for us: for a 7oz bottle- formula powder + 2oz hot boiled water in the bottle 5oz hot boiled water in the nuby rapid cool flask to cool - wait 2-5mins combine the two in the bottle and feed within 2h. if i'm out and about i take boiled water in a good quality flask to keep it hot, bottle, rapid cool and predispensed formula powder in a formula pot. good luck x