Looking into it:
For dust clouds you usually do 99% empty, not 68 or 90%; this would drop the weight by 10x.
The shape is just kind of bad, you should get the area of it instead and work off that, or reach each chunk into a simple shape and each separately and then combine.
You need to average and square the speed and distances, not every bit of dust moved the same distance at the same time, the calc kind of treats it as if the whole mass got moved the total distance when only the very edge went that far and everything else is everything inbetween. Same for the other end.
The height is kind of absurd, the pink line is more a width than a height, perspective do be a thing that exists, it's more to the side, and Mark's distance isn't even remotely the height.
Very rough numbering gets me like 6-B and Class P or above (the good news is, even lowballing it deliberately just to check the floor doesn't drop it below P) and that's before actually calcing the proper shapes and thus distance, so, it's still a good feat, but it isn't like, even half as high as shown in the OP.
I am not recalcing this btw, we're about to get a stupidly high LS and AP feat to where this won't even matter and going pixel by pixel to get the exact area and footprint of dust cloud gon take like 3 hours, not worth the cost tbh.