Idea to modify yurch nade (if there is a way to know the free volume inside a sphere/cube):
do a ratio between the free volume inside a sphere/cube located at the M67 point of explosion and the full sphere/cube volume then multiply it by say 75% and boost the radius of the nade by this ratio (and make sure the radtio stay between 1.0 and 2.0).
now if there is no way of knowing the free volume inside a sphere/cube: throw six ray (North, S, E, W, Up, Down) calculate de resulting cube (by limiting each ray to half the full cube size) and make the ratio with the full cube.
EDIT: ok, thinking about it, multiplying the nade radius by the ratio is not right (cause doubling the radius will multiply by 8 the volume), even if the nade expansion is block in some direction(s). So simply, the "power(ratio, 1/3)" would probably be best (sorry I don't know how to say the "square" with 3 instead of 2).
EDIT2: again, instead of the 4 ray (North, S, E, W) throwing 8 ray (N, NE, E, SE, S, SW, W, NW) would probably give for bettre result (and it's not really harder to calculate the volume of a cube compare to a octagonal base rectangular prism.