I even used to live out your way, Drew, when I worked at China Lake.
20% output variance is definitely ok for enlargements.
Barium Sulphate is a good coating and the traditional choice for integrating spheres, but the spectral output will measurably shift as it ages (I believe it's an oxidation issue). Not a problem for an enlarger application, as you can adjust with color filters. Even when the spectral output is critical, the coating can be stripped and reapplied to being the output back into calibration.
That's one of the reasons the industry has shifted to powdered PTFE coating. Ever-so-slightly less output (99.6 vs 99.8 if I remember correctly), but spectrally stable and just as easy to apply.
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