Foreman-Mackey and I decided at lunch that the density of his calibration grid for SDSS Stripe 82 and the radius out to which he selects stars around each grid point are both complexity parameters in our calibration model. The first sets the number of free parameters, and the second sets the smoothness. We worked out a semi-practical way to search this space, applying cross-validation to choose the complexity parameters. It is an enormous amount of data; F-M has re-photometered every point source in every run, engineering-grade or science-grade, whether the source was detected or not. That's a good fraction of a billion measurements.
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