I’m in the same boat, the controller does not display a percentage.
So I’ve been recording my numbers for each flight.
I usually fly till voltage drops to 40V (approximately 150a running). That ends up with a resting voltage, when all is cool, of 3.6 V per cell in my 6S batteries.
There is a couple of times when I had to push it a bit to get back home and ended up with a power on voltage of 36V which ended up being a resting voltage of 3.2V per cell. Don’t really want to go that low.
The thing you need to watch is that the volts drop off RAPIDLY once you get below 40V.
I fly an Advance Epsilon 6, 26. Loaded in the middle of the range.
To 42V, I get 15 minutes with 4 batts, 33 minutes with 8 batts.