Billy, 15-20% loss from the engine dyno I run to the chsssis dyno that car was tested on is pretty typical.

I think part of problem with trying to interpolate the numbers between the two, at least in this case is, they aren't using the same correction factor.
Most engine dynos are using the "standard" correction factor, and many of the chassis dynos use the "SAE" correction factor, which means they are correcting to different weather conditions, with the SAE correction being the poorer of the two, so the numbers are lower.

So, the 920 fwhp was standard corrected hp, and the 778 rwhp was SAE corrected hp.

I can't recall if there is a humidity factor in the SAE correction or not, but iirc, the temp is 77deg, and the baro is 29.38, whereas the Standard correction uses 60deg and 29.92 baro, and zero humidity.