Sepp,
We crossed.
Clearly we are thinking along the same lines.
If you look at a real manual the problem is that they are not set up for a constant N1 - they are usually set for a chosen Mach number or other parameter eg long range cruise i.e. the things of interest when flying.
My comment regarding what you said about piston engines was merely pointing out an important instructional technique - stick to the question asked. This is the instructor forum after all
Can the original poster let us know the source of this question? - Is it in the JAA question bank or is it an Oz question?