How do you calculate %xHKO?

The problem with your approach is that it's not dynamic at all. After three hits, that square looks horribly disfigured. And after 100 hits, even though it looks a lot like a bell curve, it's still not exactly a bell curve. It's just a square wearing the bell curve over itself and you'll still be able to see the corners, if only slightly. So your approach should be a shape that can transform with by using the x hits variable, not a static one.

Central Limit Theorem:

If n is greater than or equal to 25 (n being the amount of random numbers being used), then the distribution of random variables can be approximated using a normal distribution.

Sure it's an approximation, but it gets pretty close to the actual answer. This would be good to use, but I don't think our n's will come even close to being 25. 3hko's with cannon give n=12. even 5hko's give n=20. And not many people here are going to train on something that requires more than 5 hits to kill. So yeah, though it WOULD be applicable to use a normal curve as we do more hits... people in MS are more worried about the lower n's than the higher n's.