You can't.

The casing mass is gm, so if each fragment has mass gm there are fragments.

Now we need some assumptions (which are in fact invalid). Assume the fragments fly out so that they have a uniform distribution over the unit sphere, and do not fall or decellerate etc. Then a body has an cross sectional area of about sq m, a m radius sphere has a surface area of sq m. So the mean number of fragments that will hit you is about .

Now assume that the number of fragments that hit you has a Poisson distribution with this mean.

RonL