The guidelines for making up a bottle, as redcrx says, changes years ago (5+)
Nothing to do with milk going off when made up, or going sour if left out on the side but because of germs and bacteria actually in the formula powder itself. The powder is not sterile, even before you open the tin. You cannot sterilize a powder, only liquids
http://www.guardian.co.uk/society/2004/nov/19/ health.medicineandhealth
To optimise maximum drinkable milk for baby, you should make up each bottle as baby wants it with water no less than 70 as the heat will kill of most of the bacteria
WHO 2007, How to prepare formula for bottle-feeding at home.
http://www.who.int/foodsafety/publications/mic ro/PIF_Bottle_en.pdf
WHO 2007, How to prepare formula for cup-feeding at home.
http://www.who.int/foodsafety/publications/mic ro/PIF_Cup_en.pdf
WHO 2007, How to prepare powdered infant formula in care settings
http://www.who.int/foodsafety/publications/mic ro/PIF_Care_en.pdf
These links contain excellent information on how to prepare formula in todays busy life
http://www.dh.gov.uk/assetRoot/04/12/36/20/041 23620.pdf
So, put simply, to ensure maximum safety for your baby, you should follow the guidelines issued by the Gov - After all, as I have always maintained, I don't like my food re-heated so why re-heat my babi