So to clarify why I don't think it is possible for Shigaraki to have been knocked that high, we need to examine
how Star and Stripe punched Shigaraki.
From the look of her punch, she didn't actually give Shigaraki and the Nomu an uppercut, she swung her arm around in an arc from behind her body and the reason her fist appears higher than the two of them after the impact is just because her punch continued following through the motion.
Here we can see the starting position of her fist then where her fist ended up:
Here we can see see Shigaraki and the Nomu below where her fist ended up, and we can see the impact effects behind the pair of them, looking mostly horizontal. I added in an arrow to indicate though there is some spread, the majority of it is
not vertical.
Then on the next page where we see Shigaraki and the Nomu being flung backwards by the impact, we can see that the speed lines are all mostly horizontal, indicating that they are in fact being flung back mostly horizontally and not being punched straight up in the air. They were hit head-on by the punch; not punched from below.
So, their initial starting position we accept as being some 464.10 meters above the cloud layer which in turn is a couple of kilometers above sea level due to the thickness of the clouds and the height of the clouds above the sea. So judging from the angle at which they were hit, it is extremely unlikely that they were sent vertically 20 km from their starting position. The shallowness of the incline means that they'd have to be sent horizontally about 50 or 60 km away in order to reach that height. Considering that
Star and Stripe immediately followed the punch with a clap before either Shigaraki or the Nomu could recover, this just isn't feasible.
We also know it's unlikely that Star and Stripe's avatar moved far between these moments as we are directly shown that her avatar is
standing waist-deep in the cloud level as she delivered the punch and she is
standing waist deep in the cloud level after the clap when she performs her laser combination move. There is a consistency in her positioning; no indication at all that the avatar moved up 20 km.
Even more damning is that Shigaraki and the Nomu are shown to be level with her avatar (and the cloudline) when she performs the laser attack, meaning if she had clapped them at 20 km above this level, she would have have to wait for 63 seconds for the two of them fall from the point where she performed the clap attack. No matter how you slice it did not take 63 seconds for the events of
these two
pages to take place and for Shigaraki and the Nomu to be completely incapacited from the clap for a minute seems at odds with their shown regeneration speed too.
Since we have no accurate way of measuring how far vertically Shigaraki was punched upwards (if he was punched upwards at all by her attack), I think that
this modified version of the calc is more reasonable to use. A dispersion radius of 10 km a lot, but it's a lot more reasonable than 50 km which isn't supported by any of the other panels or the depiction of events in the manga.
