Attached the GlouGlou's example with parametric mirroring.
Just move "MirrorPlane" and the mirror feature will be recomputed accordingly.
That's a bit tricky but it uses a line (called "MirrorAxis" in this file) that determines the mirroring parameters. The appropriate values are transmitted to mirror feature through expressions.
In this example, I placed an "optional" MirrorPlane. Purpose is mainly to demonstrate how the MirrorAxis could be used in another design by being mapped to a face/edge/... (here MirrorAxis is mapped to MirrorPlane). It also makes the mirror plane a bit more visible.
If it's something often used, the process could be very easily automated with a macro.