Oooh hadnt thought of mixing cold and hot, thats a good idea!
Just so you can see how amny different methods there are though, this is what I do:
Boil fresh water
While thats boiling, rinse out 4 bottles and caps/teats etc and put in a sink of hot soapy water to soak for 10 minutes.
Take bottles out and give another rinse, put in microwave steriliser
Make up bottles with water first then level scoops of powder (1 scoop per oz)
Put straight into fridge to cool quickly, right at the back (make sure there are no meat products on same shelf or directly above or below)
When i need a bottle, I zap it in the microwave for 20 seconds just to take thr fridge chill off (give it a VERY good shake to disperse anf hot spots)
If bottle is needed while still hot, I put it in a jug under a cold running tap and leave the tap running - takes about 2 mins to cool that way.
Bottles are fine in the fridge for 24 hours, but once out they need to be discarded after an hour.
Hope that helps