Has anyone implemented SH exponentiation AND managed to approximate the optimal linear (OL) values for 2 or more sphere occluders?

The SH paper briefly mentioned/implied that the authors managed to fit the OL approximation to multiple spheres, which still has some artifacts with inaccurate/lighter occlusion, but is still preferable to the single sphere approximation when dealing with multiple sphere occluders, which is common in most "practical" cases.

If anyone has some idea how the fitting to multiple spheres thing is done - that'll be awesome.