This is kind of like the advertisements for aspirin that showed brand X takes effect 30% faster. What they don't show is that both take effect within 10 seconds. Make the circle small enough and extend it as far as possible so it overlaps the thick grid lines and ignore larger areas of effect. Also assume any square that has half it's volume filled is fully affected which is the opposite of the general rule.
When I do it? It looks a lot like:
Are those squares that have less than half filled considered affected? I would say no. I would have to thicken my grid lines and then extend the circle to the outside edge of the grid line like you did to even make it close.
Or take a look at a circle that actually has the same size as a fireball
Those squares at the corners aren't even touched, much less several squares that only have a tiny portion included.
On the other hand if you want to use non-euclidean geometry, feel free. It's one of the options listed.