Dan,
Not being an enginer, I am reluctant about giving out any definitive engineering advice over the web such as an actual calculation for someone else's project, especially without seeing all of the conditions in the design. I advise you consult an engineer or the Timber Frame Engineering Council. That said, if you want to learn to figure this yourself, you could consult the books that Mike recommends, especially the tension joinery articles in the 1st volume. Design values for the various woods are also there, in the NELMA grading manual and the National Design Specification Supplement published by the AFPA. Calculating the new centroid based on the removed material in the post can get a bit complicated, but I would recommend some of the books in the Parker/Ambrose simplified structural design for builders series. We also cover much of this material, and the calculation I used, in our Timber Frame Design and Joinery Decisions course at Heartwood. I'm reluctant to just throw this calculation out there because there are many underlying assumptions and data that need to be considered, and the calculation needs to be done with a background understanding of basic engineering, such as where to find design values for the wood. In our design course we have a week to build up to this sort of calculation, but it's too much to do here.