Side Wing Odd Roof Angle

When I first started making buildings, I used the ol’ trial-and-error method to cut the roof pieces when attaching a building wing to the main building. Eventually, I added a modicum* of intelligence to my brain and figured out an easier and more precise method.
* a bit larger than a smidgen I've been told.

The problem: Just what IS that weird angle?

Since almost all of my buildings have symmetrical ends I made a couple of measurements: (A) is half the width of the end and (B) is the measurement from the wing’s peak to it’s side wall.

I then cut two overlong pieces of MDF (one for each roof piece) at the finished roof height… usually (B) plus 1/8” or so for overhang.
 From the corner on one piece I measure (A) across and (B) down and make a mark. I draw a line from this mark back to the corner and now have that mysterious angle. I cut along the line on my scroll saw (easy and accurate when using my new handy dandy Adjust-O- Matic jig), then cut its mate. Next, I sand the underside edges of the two wing roof pieces where they’ll meet the main roof for a nice, tight fit and temporarily tape them together on the underside.
 I make the main roof, gluing the two pieces together with gussets and notching where it meets the side wing wall. I place it in place.
 I temporarily set the new wing roof in place and mark, then cut, the final lengths (as indicated by the dotted line, again remembering the overhangs).

Finally, I glue the side roof pieces to the main roof. After drying I glue some short dowels where the two roofs meet on the underside (for added strength), remove the tape and glue in a gusset near its end wall to connect the side wing pieces permanently together.

Kitty Wankus asks “What about roofs that AREN’T symmetrical?”

A: Just about as easy. Lightly mark a vertical line from the main roof peak down. (A) is then measured from that line to the edge. And if the side wing roof end isn’t symmetrical the (B) measurement will be different for each roof piece.