At least keep it at 35 meters or even higher. I always take at least 10 meters distance. Just make sure you can land it safely and it doesn't do RTH or you might get in the way of someone landing a kite.
Ah btw the height calculation is very naive. 25m is average kite lines lenght, fine, but even excluding the stretch, the kiter waist will stand at least 1m from the water, plus waves would raise him a couple of meters and... the C-Kite protrudes at least a couple of meters when powered. So 30m would have been the very minimum height to fly... but still not taking into account other variables, such as jumps.
I am both a drone pilot and kiteboarder. This was very bad and dangerous drone flying on your part. The length of the kite lines and the extensions are immaterial. Kiters like to jump and jump high. You should either stay over shore or keep your drone much higher when flying over kites IMHO.
Great video! Nice work! I love videos like this.
To soothe all the worry warts, you just need to put a screen in the beginning that says
"No innocent Kite surfers were killed or injured during this flight"