Is it possible to change the zoom values found in the Navigation Toolbar?
Tools -> Options... -> User Interface... -> Zoom magnification factor:
Do you really want to ruin economy only to save the planet?
Though that doesn't change the preset values in the drop-down list. Those are hard-coded.
(I'm not sure what is ultimately wanted or why.)
If you want a scale not in the drop-down, just type it in. Of course your active component must be a projected one..
And you don't need to type in the whole ratio, just the divisor. E.g. for 1:25000 type 25000 or even 25e3.
Zoom magnification factor controls zooming steps for the Zoom_In and Zoom_out button and - more important for everyday use - zooming by mouse wheel.