I used one with my 24 weeker and never had any issues, it was a godsend x
I also just stopped the water flow early by pressing the button when he was on smaller amounts x
I use my prep machine for my little girl who is almost 5 months (10 weeks corrected) and I basically make up the 120ml and discard what I don’t need for example if her feed is only supposed to be 90ml I pour away 30ml after making the full 120. That way I know its been mixed and prepared correctly. I also know it looks like it is adding more water than what you set it at but thats to do with the formula being added as when I checked it without the forumla it poured the perfect amount so I would always let it add all the water so that the formula is correctly diluted (hope that makes sense!) I found out that its to do with the hot shot that it does as to why 120ml is the lowest volume it will make! But it has honestly helped so much as I dont have to think about boiling the kettle etc each time! I’ve had no problems with doing it that way and were on a prescribed formula too! X
I have a preemie. We boil a flask of boiled water we keep for 12 hours abd turn change and do a flask of cooled boiled water we do the same with. We do a shot of boiled- dependant on volume and temp they like it, add the formula, shake it then top up with the cooled. Basically what the perfect prep does . We do a day flask and a night flask abd have had no issue x