The real supplied energy for some device is not equal to the energy of a battery, right?
Here's the charge/discharge curves for that battery:

It doesn't sag to 3.0V until 100% of the capacity is discharged.  Note you won't get that in real life, but you'll likely get 90% while it's capacity is still fresh.  Here's the full datasheet.  Also, don't forget to include parasitic voltage drops.

I'm not sure what you're hoping to gain with any energy calculations.