Sheldon Brown's page:

Fork Lengths by Damon Rinard

He also uses the rear axle as the pivot point when changing a-to-c. His numbers tend to agree that with a ~10mm longer a-to-c, the head angle will decrease by about .5 degrees. So that 45mm to 50mm fork won't quite be as drastic as a 5mm change in trail. It *should* be a bit less. I just don't know what that number is exactly.

Anyhow, I appreciate the conversation on this. It's helped me have a bit more understanding about what's going on with front-end geometry.