I have built and flown 7 Aeromasters (60-90 size), two Phaeton90's, two Phaeton40's, three Tiger Moths, several others and all flew best with the upper wing at about -1.5 to -3 degrees incidence relative to the lower wing. I have made several experiments where I was able to vary the incidence on the Aieromasters and Lazy Aces and found that this was always the best setup for any airobatics. If you can, build the model in such a way that you can reliably vary the upper wing incidences and do your own experimenting. You will find a point where the plane suddenly grooves just the way you like it.