We consider the optimal makespan C(P, m, i) of an arbitrary set P of independent jobs scheduled with i preemptions on a multiprocessor with m identical processors. We compare the ratio for such makespans for i and j preemptions, respectively, where i < j. This ratio depends on P, but we are interested in the P that maximizes this ratio, i. e. we calculate a formula for the worst case ratio G(m, i, j) defined as G(m, i, j) = max C(P, m, i)/C(P, m, j), where the maximum is taken over all sets P of independent jobs.