You guys must work the night shift.
Stephan, Bill's right in that I'm not looking for an absolute measurement, your card/black box experiment actually gives me the tool I need to figure this out in general.
I can see that in the situation I had at Chimayo I should have expected; 1-flare to trick my N90s's meter into thinking the scene was brighter, and 2- flare to lower the overall contrast of the film as a pre-flash would do.
With those two expectations I could looked at the scene and known to add "some" exposure knowing that flare was probably tricking the meter and knowing that it was a mixed scene, compared to your card test, I would have known to add "some" exposure to "refill" the paper's curve because of the reduction in contrast flare was going to provide.
As a side note, this also reinforces my preference for incident metering.