I recently added overhang's to house both gable end's and the rear. the house front was already there.is there a basic rule as far as how to figure out boxing the ends for running facia trim and soffit on a 4/12 pitch?

For trimming the rafter ends measure out from the outside wall and pop a chalk line along the bottom then use a plumb bob to mark vertically at your mark on each rafter. The cuts don't have to be exact but the closer they are to flat and vertical the better. If the roof is fairly square you can use the edge of the decking as a guide instead of measuring and work out any differences as you go.

Here is a link to some soffit and fascia details:
http://www.extremehowto.com/xh...asp?article_id=60487