Now:

R_w = KW_a^1/3

where K = the "weapons radius" in feet for a 1-MT attacking weapon. It is constant for any particular target but varies with hardness of target;

W_a = yield of each attacking weapon in megatons.

Then:

P_s = 0.5 exp [N_aW_a^2/3/N_a(K/C)²].