3.16%/Hour is equivalent to 1 Day, 7 Hrs, 38 Mins and 44 Secs of Standby with 3G on if I suppose? Now If I may suggest you a key, when you work on any app and then close it by pressing the Windows Key that app is running in the background. I can vouch for its standby with 3G on and running apps closed by long pressing <- and exiting all apps by clicking on X. That ways my NL520 drained only 7% in 7 hrs Remaining Battery Life 93% & Time since last charge: 7 hrs. The % you say is shown in Battery Pro + developed by Nokia itself. the % is average of last 48 hours usage. So Don't fret my friend; closing all the apps by X does not close them rather like OperaMini the tabs and status are in standby mode. They will continue from where you left off. And also you cannot open more than 6 Tabs at once thus saving resources in WP8 Black.
My Battery experiecne with L520 is as below:
3g on, lockscreen times out 30 Sec, brightness control set to low(with Automatic Adjust-On), battery saver on(Conserve Until Next Charge), display - sunlight readability On, - battery saver brightness On, lumia colour profile is Neutral, touch sensitivity Normal and double tap as well as WM keys vibrate set at OFF.
Day 1 - Charged 100% at 10:00 am
Calls 75 mins(Volume 5/10) (approx), Messaging (5-6), Nokia Music(on Loudspeaker) 1hr 20mins, Internet 30 mins, Games 20-25mins
Day 2 - @ 10:00 am Battery at 42%
Calls 45 mins(Volume 5/10), Messaging (3), Nokia Music(on Loudspeaker) 60mins, Internet 10 mins, Games 18-20 mins. It died out at 11:15 pm (approx). Extracted the most juice out of it.
So proud to say that I have extracted almost 37 hrs from my NL520 - Impressed with the standby battery conservation.
PLEASE NOTE that I've not installed WhatsApp on my NL520 as it constantly seeks Internet Sync. Though I have 3 email IDs set-up syncing at manual interval