Not sure if this will help you but what i used to do (and will when this bubs arrives too) is to boil a keetle and when cooled would fill the bottles about 3/4 to where i wanted them and then put in fridge. Youll have to amend this ratio when you know how much milk you are making so its trial and error to start (thats why you always check heat of bottle before feeding baby)
When baby needed feeding id then boil a fresh kettle of water and top bottle up with the hot water to bring it to right temp before adding the milk powder
It always mixed well and i never had any problems with it like that and it was a good routine for me to use.
Boil a kettle, fill the bottle to the correct amount, let the water cool. Put in fridge. When the baby needs feeding, insert the baby milk, boil kettle again and put hot water into a bowl, place bottle in the bowl and allow the milk to warm up.