question for bill:  is the shadowcatcher in the linked rdna thread the same one included in your tomcarscene.pz3?   tomcarscene shadowcatcher gives me good ground shadows with hdri only, but darkens the entire area covered by the plane, for which I don't know how to calibrate.  the rdna shadowcatcher gives me a white opaque plane - not sure how to calibrate that either.

I'm guessing that once I get a render with a shadow over the bottom of the hdri sphere, I click on darkest area (black) to calibrate shadow colour and click on lightest area (light grey) to calibrate lit colour.