According to the
official description : "⁴A single charge on the eReader lasts up to 53 days, based on 30 minutes of reading per day with the front light at 10% brightness and WiFi and Bluetooth® wireless technology off."
53/2=26.5. That means total of 26.5 hours of battery.
100/26.5=3.77 which means for each hour of reading the the battery should drop around 4%.
But we know every company over estimate their battery life. So assuming you're reading font-light turned off or below 10%, the battery should drop approximately (4 to 5.5)%/hour (i.e. drop 1% every 16 to 11 minutes respectively)
That's how I measure the battery performance. Am I correct in my math?