Also, I made a nice plot of uncertainty in proper motion (in km/s) versus distance(in pc) from some listed LSST uncertainties (in mas/year). This should be a simple unit conversion problem, but for some reason both axes run from large negative numbers to 0 instead of from 0 up. It seems like the problem occurs where I initialize my vector of distances, because the only thing I to to them is multiply by positive factors but they turn out negative. That'll give me something to futz with tonight, at least.
*update, it turns out I was doing scientific notation wrong in IDL, which was confusing it. I'd also forgotten to convert from mas to radians, which explained why the errors I was getting were so high. Now things look somewhat more reasonable.