scarf location

In a 28' plate atop three posts--the plate carrying common rafters of a porch addition--where is a scarf best located?

David, Jack Sobon's book has some informative advice on scarf location (Timber Frame Construction). Another alternative to his suggestion which is often seen in older buildings as well as new construction is to place the scarf over the center post with the post tenon extending through both halves of the scarf. I'll be watching to see what others have to say. This connection may not be extremely critical depending on the frame design of the roof and the loading. Good luck.