to accurately meausre crank hp you'd need an engine dyno, not a common dyno that you've used. Crank hp is always calculated and is just connected to too many variables/settings on normal dynos to be reliable. Hence all that should be quoted is whp, and again hope that the dyno settings are correct in the first place.
its calculated by measuring transmission losses on the dyno, its not as if they just apply some magical formula to it. If other cars using that same dyno get to their given hp (e.g. the TDI engines from the VW group) and others dont (e.g. the Z, or the RX-8 which apparently is a complete disaster on that level, or the S2000) there are some conclusions to be drawn
But i agree, the most accurate method is by using an engine dyno.
it takes into account the losses on the dyno but it isnt really 100% accurate when it comes to calculating the crank figure as things such as tyre pressure, transmission losses, oil used in diff etc can all take effect. Also by simply moving the IT probe you can get wildly varying results.
It is a good measure of a mod, rr it before the mod then take back to same rr after the mod to see the change.
Saying that, RR days are good as it gives a genuine comparison on the same day same RR set up against other cars even if the figures are not as expected it gives a good finger print