Hi Speedy,
thank you for your nice calculation. Mathematics was one of my worst subjects at school. But, I don't think that you can calculate 90% kill radius like that, without having much more data like : weight of 1 fragment, velocity(here you could enter some reasonable value), dispersion pattern (here also you could enter some reasonable value if its a ballshape). Normallly the 90% value is established in a fragment garden by tests. For a single hit to have such an effect, the fragment would have to be quite large and to have a very high velocity. So, I think that you can not calcualate this value so easily.
Regards,
Bellifortis.