You won't ruin the battery at 13.8v. In fact I have no proof my method is better than the next. It's just what I do and the batteries seem to last.
The concern is per cell voltage. Since the H doesn't monitor individual cells, you have to assume there is one cell that is lower than the others and the H force lands at 13.8 doesn't it? That's still 3.45v which doesn't concern me in the least. If there's a cell that's 3.0 the battery is toast anyway.
To compare flight times between batteries, there needs to be a standard test pattern in similar environmental conditions. What I do is move the slider to Turtle mode or a bit higher and go full throttle in figure 8 patterns to keep the speed consistent. Actually I also turn GPS off. Don't that if you're not confident to fly without it!!! Then a standard hover only test.
What I'd be interested in is how many mah's each battery takes after a hover test about 3 meters off the ground, then land as soon as it hits the last warning. Allow them to cool off for 15 minutes then recharge.
I see no need to finish my battery case project. You guys have done well. Plus my H is out of commission at the moment.