We are framing up a new little vestibule tomorrow.  Take a look at the attached pic.

The circle shows where the existing barge rafter, faced with a rake board, will die into the roof as we will extend it.

For our little gabled vestibule, we will cut the barge, hang a ridge to the rafter that is back from the barge, and roof frame over our new walls (blue is new).  I am thinking that the existing barge will be cut so it sits right on our new sheathing, and then we should step flash our shingles to that face, then cut the shortened rake so it sits up clear of the shingled surface.  Our cut rake end should be well sealed with primer and paint, plus its back side.

Sound about right?