I'll try to explain why I don't think
Kachon's calculation should be used.
This is what is being used to find the diameter of the explosion, the diameter being that red line which in comparison to the calculated size for the city gets it to be 18.5 kilometers in diameter.
Now, this is an omnidirectional explosion, so if we draw a line going downwards from where the explosion supposed happened, we get this:
The vertical extent of the explosion actually reaches off panel, it'd encompassed the entire city easily. If the explosion detonated that high off the ground and was as big as what has been scaled, then the city should've been destroyed.