I can see the bottle inside the squonk hole. When the bottle gets about half way done, I change the battery. I top off the bottle each battery change if I happen to be home and if not, then the other half uses a battery. The batteries are usually somewhere between 3.65 and 3.8 when I remove them after they have fired ~half a bottle of fluid.
Since I vape different times and amounts each day, this method never fails even if I chain vape. Half full bottle = battery change. It took a lot of metering to figure this out, but it works well for my 0.8 to 1.1 coils.
Thanks for this @vsummer1. My REOs are a few days old and I am observing a similar thing. Drop off in performance and time to change the battery when the 6ml juice bottle on the Grand is about halfway through. I have mechs but since I got the Reo its the first time for me using a mech for continuous 'workhorse' vaping.