What's the best way/time to factory reset my iphone?

I have an iPhone 5 and I updated to IOS7. Since the update my battery has been draining around 5% per minute that I use it. My brightness is perpetually on the lowest setting, I don't have bluetooth, air drop or notifications on. I don't use location services and I'm probably one of the most conservative people I know when it comes to my iPhone's battery. Will factory resetting restore my phone to IOS 6? And when is the best time to do so (eg. When the battery is at 1%, 50% or 100%)? Does that even matter? Thanks in advanced. It's really frustrating when my phone used to last anywhere from 12-16 hours and now only lasts 5-8 hours.

All i can tell you that itunes won't let you restore your phone to ios 6. You will have to use different method to downgrade if apple is still signing shsh blobs or if you have them saved you can downgrade but not through itunes