to mirror an object set the scale to -1 in one of the axis parameters. You can do this by selecting the object and using the text entry boxes down the bottom of the screen.
It's a percentage of the global bounding box of the object. since your chamfers are pretty big compared to the cube, you need to set this to 0.1 (10% of the size of the object) indeed.
nonono I want it to work the other way around haha. See I want to run a simulation on an object and have the bones follow some of the verts on this object.