When I have a structure that needs the total mass to equal the known mass of the structure, but the shell and/or solid elements in the model add up to some fraction of the known mass, say 0.8 I adjust the density of the material in Engineering Data by multiplying the true density by 1.25 in this case, so that the mass of the structure becomes equal to the known mass. This distributes the missing mass over the entire structure in proportion to the existing mass.