Experience.
Also, practically every manufacturer of things that use Lithium batteries, recommend that you charge cycle a couple of times when you get a new device, so it can "calibrate" it's battery meter. Well, when you do a major update like Portico, you tend to loose the calibration information for some reason (lazy programmers?) so it's good to do a charge cycle.
With my 810 after the update the battery meter was saying I could barely get 12 hours off a charge. Previously I would say I could get just over 24 hours off a single charge (I use my phone a lot during the day). I decided it was time to do a charge cycle. i made sure the phone had a full charge, I then turned on every feature I could (Bluetooth, NFC, WiFi during lock, long timeout for lock screen, etc.), turned off the "Battery Saver". Then I queued up all my songs, plugged in some cheap headphones and cranked up the volume to 30 (without wearing them), and surfed the net till the phone powered down (it took awhile, but it powered down right before I went to bed luckily). Proceeded to charge the phone overnight.
In the morning, I went through and changed my settings back to what I had before, Bluetooth off (I don't have any other Bluetooth devices), NFC off (don't know anyone else with a NFC phone, nor do I have any NFC tags), WiFi off during lock (I have unlimited cellular data, so no big deal), lock screen timeout back to 1 minute. Things are back to being better than before. The phone now says I could get around 36 hours of use before a charge is required. Last night when I went to bed it said I still had 65% charge left (it's never been that good) before I plugged it in.
So try it, it only costs time to do it.