Nice job working this out!
Ran across this recent report from Jack Ganssle on ultra-low power ...
Hardware and Firmware Issues in Using Ultra-Low Power MCUs, Dec 2014
Lot's of interesting info on sleep current vs. sleep time, technique to check voltage as you ramp up frequency/clock from wake-up, battery droop, choosing de-coupling capacitors carefully, and more. He emphasizes "successful low-power design requires careful analysis of every component used ...", which folks have clearly flushed out here.