So how do you use it?
To judge power on time in flight, use it to insure the flight starts with a full charge. It tells percentage of charge in the battery. Then fly the plane the way you usually do, but limit the flight to less than that recommended 6 minutes. When you land, hook up the meter and read out the capacity remaining. Say you flew for 5 minutes and have 50% capacity left. Figure from there what you want to land with next flight on that battery. Of course, we seldom fly sailplanes with throttle most flights so.....................
When I'm flying my sailplanes, the power is only on for the climbs. So I figure out how many climbs there are in a battery. Looking at the usual 97-98% start charge, and say 5 climbs, the charge remaining tells me how many climbs are remaining. And I don't have a single battery that won't do way more than 10 climbs. And the last few months of flying, I can't remember any flight that had more than 7-8 climbs in it. The last few months, none has had more than 3-4.
The meter mentioned above is called a wattmeter, and that's an excellent and valuable function. It's worth the money just doing that. We can easily and safely fly glow fuel engines without having a tachometer, but there is no way to fly electric without risking motors/ESCs/batteries if you don't have a wattmeter and battery tester.