Ah, now you got me and things are going to get complex. I have to admit that I've never seen a plan where the designer shows the downthrust angle with reference to the wing's airfoil center line. It's always given as an angle to the ... er..... FRL ..... er.... which of the smilies is the "embarrased" one ?

That makes things a bit complex doesn't it? Even though the angle that COUNTS is the one between the wing and the thrust line no one gives it that way. I practice the angle between the wing and the thrustline can be between 0 for a hot pattern model or up to 8 or 9 degrees for a rudder only old time vintage model. Most trainer models like a Goldberg Eagle will be around 4 or so and something like a Great Planes Sportster would be perhaps 2 to 3 tops.