You're on the right track.
The main wing generates downwash as a by-product of lift generation; the amount of downwash is broadly proportional to the lift created. (Think of the momentum change in the airflow as being the 'equal and opposite reaction' to the lift force; a crude but useful way of thinking of it.
If the tailplane were not behind the wing, it would be in the undisturbed flow field, and the angkle of attack at the tailplane would be the angle of attack of the flow (alpha) plus the angle of the tail relative to the fuselage.
Behind a wing, however, you have to allow for the downwash in determining the flow field. So the tailplane angle of attack is equal to the fuselage angle of attack, plus the tailplane angle, minus the downwash angle.
As the downwash is a function of the fuselage angle of attack, this creates a situation where the effect of a (say) one degree shift in angle of attack for the plane overall may only cause a half degree shift in tail angle of attack, as the downwash cancels some of the change.
Allowing for this effective change in tail angle of attack is important, as it is often a large effect (50%, or so, is not unreasonable for quite normal configurations).
It's a lot easier with a diagram!