How does common-mode voltage affect a differential amplifier?
Op-amp used: LM2904.
First, one must define a schematic that can be used for measuring.
And then you can "calculate" or "simulate" to see what happens ...
EDIT: added Maple sheet (EE&O).
Schematic : (EE&O) I may suggest this one, with your specs voltage inputs. (Vmin -> V3, Vplus -> V4)
Or this one ... but Gmc is not equal to zero.