Hmmmm -- actually, since stops are ratios, I believe after the extrapolation, you'll still only be off a quarter stop. But the basic situation is true that there are enough variables tangled up here that some simple tests are likely the quickest way to get there.
Originally Posted by Rich Ullsmith
The fudged metering works pretty well for me with film, but since paper doesn't normally spec an ISO and may have some spectral issues as well, it would be necessary to figure out an ISO from tests to get any real precision.
Myself, once I start pinholing, I tend to toss precision out the window.