Kevin,
You can eliminate the "...then multiplied by ((joist depth/tenon depth)squared)..." factor if it's a gradual reduction, which would be the same if it was a straight 4x4 all the way. Shouldn't be "squared", tho. That may be a misprint; I'll check the book next time I'm at the Heartwood library.
Again, that factor is a penalty for having a square notch at the reduction.
