I adjusted the calculation using the height you provided for the mountain. Since the fireball is behind the mountain, the fact that the mountain is closer to the viewpoint simply would causes the calculated size of the fireball to be smaller than it should be. In other words, the mountain not being near enough simply makes the calculation an low-ball. So I think we can use the mountain as a reference because it doesn’t inflate the calculated value; on the contrary, it causes it to come out lower.
You could use an alternative method. That person transformed all the humans around her in ash, so you could do one of the following: