As I said, this feat is still pretty debatable, considering how these guys aren't meant to be superhumans to this degree, and haven't done anything which gets their speed around the level. The most arguable thing would be that Sawaragi isn't even looking at the scene. This is similar to a Tokyo Revengers feat I remember wasn't accepted due to similar reasons, if you see
here, the feat is almost the same (except for the fact that the supposed outspeeder here also kicks the gun out of the shooter's hand, and the accepted calc of that used 1 meter as an assumption, instead of roughly 3 to 9 meters), the person who is going to get shot isn't looking at the shooter, so them being shocked doesn't justify it. Although it's current status, I believe an anime adaptation can easily prove its validity, and I am personally interested in it, but calculating it now doesn't seem much fruitful.
As for the calc, I don't see anything wrong with it if it satisfies the context of your assumption.. About the bullet speed, you should use 90 m/s as they weren't using real bullets, but rather paintball guns (
source for the speed).