The way described on the tins is the officially safe way to make formula so what to do depends on your level of comfort. The water needs to be at 70 degree or above to kill any bacteria in the powder. If you keep freshly boiled water in a flask and replenish after every feed you make you don't need to wait for the kettle to boil.
Charli - if you are making bottles in advance then they are ok for 24 hours BUT really you should be adding the powder to the hot water then rapidly cooling under running water then putting in the fridge (right at the back not the door). The trouble with adding powder to room temp water is any bateria aren't killed off and if you are leaving them for 24 hours before use that is quite a long time for bacteria to multiply and milk is an excellent medium for bacteria

.
We had to add our reflux powder to room temp/ cold water but we used that feed straight away. We did pre-measure the freshly boile water into sterile bottles and kept them for 24 hours. It's just as quick to add the powder to room temp water and warm (or offer it at room temp, some like it that way) as it is to pre-mix and warm to be honest.
Don't forget too a freshly made bottle is good at room temp for 2 hours BUT should be discarded by 1 hour after it has started to be drunk from because of the bacteria that washes from the baby's mouth back into the bottle.
I know it's a pain but I am much happier being safe rather than sorry!
http://www.dh.gov.uk/prod_consum_dh/groups/dh_digitalassets/documents/digitalasset/dh_124526.pdfHTH!
Laura