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Well, it all depends ...
For a given focal length, the farther the lens is from the focal plane, the more magnification.
For a given distance (extension) the shorter focal length lens gives more magnification.
One of the thing it depends on is lens construction. The "optical center" of a lens that you
want to measure to depends on lens construction.
As a concrete example, I am working with an older (FD mount) 50mm/f3.5 canon macro lens.
Built into the lens (as is the case with many macro lenses) is a fairly substantial "hood", and
when the lens is reversed, this actually places the optical elements at a fair bit of extension
from the camera -- sort of like a built in extension tube -- so this lens reversed might give
similar magnification as a 28mm reversed and mounted directly on the camera.
But I never mount them directly on the camera -- in my case I am using a nice old FD series
belows as an infinitely variable "extension tube" which I like a lot.
But the thing to do is .... try things out !! It isn't all about magnification though, at least it
certainly isn't once you find out ways to get the amount of magnification you want.
I am in the midst of trying all the different lenses I have to see which give me the best results
in terms of resolution, lack of flare, color saturation, contrast, overall image quality.
The main things is to experiment and try things, expect to be surprised.