Following the guidelines, it's boil the kettle, wait half an hour so the water is arounf 70 degrees, add the powder, cool further then use.
There are a few ways to do it, and it really depends what you are happy with.
You can make all the bottles up and put them in the fridge, and use them within 24 hours. (What I have always done, and have never had any problems)
Put some of the water int the bottles in the fridge, top up with hot water when adding the powder, then use.
Put the whole amount of water in the fridge, add powder and heat. (But you'd have to heat to 70 degrees, and allow to cool again...not sure you can guage 70 degrees to kill the powder 'bugs')
Hope I haven't confused you more. My steriliser only keeps bottles sterile for 3 hours if used in the microwave. But cold water sterilising for 24 hours. The way you describe, you are not killing off the potential bugs, and it really wouldn't matter how long the kettle cooled for as the water isn't being used there and then? xx