Hi
I used to make up the bottles with just boiled water in and leave them at room temp then add the formula when needed - the Avent formula divider is a God send if you haven't already got one because you can premeasure the formula and just whack it in the bottle when needed.
If you do this make sure that you swill the bottle around a bit first before you shake it, otherwise the dry formula gets stuck in the teat and its a pain (DH discovered that one, must be a man thing).
HTH
love