Here’s a little excel work showing how long the battery should last using my measured currents. I adjusted for your average uptime (looks to be about 20 seconds, i.e. it was taking a while to connect to wifi probably) and I also tweaked the battery capacity down to 450mA so that the time of the first test would match, about 12 hours. Then if you look at the rest of the tests, they match up pretty close. I wouldn’t be surprised if your 600mA battery wasn’t being fully charged, and had ~450mA of capacity 
Average current calculation:
Runtime calculation:
What’s interesting is your 900 second test is kind of the outlier in the bunch. 600 and 1800 seem to match my chart pretty close.
BTW: the Core did sleep fully for 30 minutes (1800 seconds) and did not wake up early or anything.
So now if we keep doubling our sleep time, we’re sure to find the point at which there is a true diminishing return.
Aha… it’s when the Wake Time and Wake Current is so diluted by the sheer amount of time that the core is Sleeping. The Sleep Current dominates and you can’t do any better than Battery Capacity / Sleep Current = Runtime Hours.