Here's another way to put it:
Shooting with RED One rated at its native sensitivity and monitoring Rec709, if you put 18% gray at 45% IRE, you'll capture 2 stops over 18% gray, plus some headroom.
If you underexpose from there by 1 stop, you'll capture 3 stops over 18% gray, plus a little headroom.
Underexpose by 2 stops, you get 4 stops over 18% gray, plus headroom.
Want to match Panalog? Underexpose by 3 stops for 5 stops over 18% gray, plus headroom.
Want to match a hypothetical "perfect" film stock? Underexpose by 4 stops to match the Cineon spec's 6 stops over 18% gray... you guessed it, plus some headroom.
And then put the results into your grading system and see if the noise levels on your gray card are acceptable.
This is what I'd do if I had a RED One, a pretty girl, and a window!
EDIT 080223: I added the italic text at the top in response to David Mullen's post on the reduser.net thread that's been running parallel with these posts.