Consider a plane that has, with respect to a datum line, – 3 degree thrustline, +2 degrees wing incidence, and +1 degree stab incidence.

Now consider the same plane, same datum line, but we change all the angles. Now it has 0 degree thrustline, +5 degrees wing incidence, and +4 degrees stab incidence.

The last example actually describes the S.E.5a (in the picture – note the stab is shown at 0 degrees, but it was adjustable, and rigging diagrams had it set to +4).

These two planes have exactly the same downthrust, not with respect to the datum line, but in actual flight. If we don’t agree on the simple geometry of this comparison, there is nothing more I can say.