Nils,
The included angle is the angle on my jig/fixture to set the incidence of the tail. This is 1/2 of the angle between both tails or 140 degrees. Sorry for any confusion. .There is a direct relationship between the angle and the projected areas of the horizontal and vertical stabilizers. I did a lot of research
and ended up making various tails out of balsa sheet to experiment on my sub-scale models. It worked out just fine.
Regards,
Art ARRO