The incidence is intende to be in relatin to expecte fuselage level flight, and downthrust is a totally separate issue.

A plane's wing incidence is designed to have the plane level at normal cruising speed and not gaining altitude. If the plane is intended to fly fast, it will have a low incidence. If it is intended to fly slowly it could have a fairly high incidence. (I've seen 8 deg) the incidence also gives you an idea of the speed range of the plane. A high incidence will indicate top speed and stall speed are supposed to be close. (some pre WWI planes, top speed was within 5% of stall speed.)

The difference n angle between wing and tailplane (Decalage) affects pitch stability and CG range. If the angle is large and wing + relative to tailplane, the CG range is moved forward. If the andle is -, the CG can go as far back as 60% of MAC of the main wing with a "normal" appearing aircraft. (old FF models, this was common. The tailplanes lifted, where its more common for us to develop a downforce with the tailplane.)

With a far rearward CG, decalage angle and CG range become VERY limited, and just burning off 1 oz of fuel could make a .40 size plane uncontrollable. (FF models rarely burned more than 1/4 oz durring a flight. The 1930's/40's standard fuel tanks wouldn't hold 1/2 oz!)


BTW... I have a relatively modern trainer, the "TameCat" by AlTech. Total downthrust angle is 9 deg. The wing is at 0 deg incidence, the tailplane is at + 3 deg (partialy lifting tail!) CG is at 37% MAC. Great trainer... but a bit heavy. Too bad they quit making it. Its nearly indestructable, and looks a lot like a F-14 Tomcat.