DJI spell out the way they come up with their Max Flight Times.
The problem is that people ignore the details.
Battery 96% to 8%, climbing to around 300 ft and circling at unknown speed in unknown winds is not the same as ..
Measured by the...
All this worry about RTH altitude control malfunction is easily avoided by just flying the drone yourself. Better to be the PIC than a spectator.
If you lose position awareness for the drone, it's dirt simple to follow the rubber band line on...
You'll be fine.
All my flying is done over the ocean and I've flown thousands of miles.
What you're planning has no more risk than flying in an open playing field.
I would have no hesitation whatsoever in doing what you describe. I'd also be comfortable flying six feet above the lake en route to the location from which you want to shoot the photo.
Just don't rely solely on the altitude indicated on the...