How can the maximum strength of aftershocks after a major earthquake be calculated?

Are there simple rules of thumb? Like maximum strength of aftershocks is 70% of that of the major quake. So for the Sendai 9.0 quake it would be 6.3

Or is there a more complex rule of thumb like

Within X weeks the likelihood of strength Y aftershock is Z.

So for the Sendai 9.0 quake within 6 weeks the likelihood of a 7.5 aftershock is 40% for example.

I’m asking this because of the Fukushima situation. What would happen if a 7.5 aftershock hit the region right now? Maybe the worst-case scenarios do heavily depend on the maximum strengths of aftershocks as well.