The stab incidence, the main wing decalage, and the thrust line are measured against the "datum" line. This is a line that travels down the center of the fuselage that when measured represents the fuselage having 0-degrees angle of attack and 0 degrees yaw. On some high performance gliders the datum line gives the fuse the appearance of being nose down, in most other applications it would be considered straight and level. Hope this helps.