So looking at it I think I was right but we run into a different issue now
All this tells me that City-A is located in the eastern portion of Region-A, which is why the spread shot shows the coast line of City M, why they went to City-N in the fight and why they could go from City-H to City-A since there's just a small buffer between N and A.
Overall I do not believe the evidence provided indicates that Region-A was nuked completely. If anything it would be a large spherical area around the eastern portion of Region-A, the northern portion of Region-M and the southern portion of Region-N were damaged by the volley. So I'm still against the current calc assumptions and I'm not sure how you'd be able to even calc it since the targeted area
probably looks like this which can be accomplished with the current assumptions with the ship's height.
It's chapter 178-180. It's pre-Ninjas and from the Tornado fight, though that was like three years ago now with all the redraw delays.