Spherical Harmonics and Optimization

Hi, All --

I am trying to use spherical harmonics to represent a surface in 3D. The worksheet calculates through without error, but the results are off by ~24 orders of magnitude (for 12th degree Legendre polynomials). I note also that the magnitude of the answer increases with the degree of the polynomials.

Anybody have any ideas? Maybe there's a better way to approach this altogether?

Thanks in advance.

Matt