We also see no disturbance in the sand in the resulting shot due to the shockwave which was shown to produce heavy winds. It's animation and obviously not gonna match the blast effects perfectly.
I'm not talking about disturbing the sand dune itself, my apologies, I meant the sand flying across the air that we see reaches Katara and the others.
He's blows sand outward and that sand/wind doesn't reach the sand dune you're using in the time we see it, implying a large distance between. I believe the sand dune method varies too much, and the cloud being in the background means the measurements a low ball.
Calling something out as an animation thing isn't the greatest, it doesn't matter since that is what is shown to us and is what we have to work with. The sand he blew didn't reach the sand dune in the time we saw it, so I can only infer that the cloud is a good distance away in the background.
Of course there are exemptions, it all depends on the scene. But I don't see how this is one of them.
The horizon isn't perfect yeah, but I don't think it's by a drastic amount, and I believe is a more stable method to use.
From what I understand the size your using is the average between the two numbers, since the pdf doesn't mention an average size for sand dunes. I'm not against such a method, I've done the same myself. However I believe the horizon method is more concrete here than what you have.
TLDR: It's a good lower-end, but not the best end in comparison to Dale's method.