Any faster way to mirror objects or groups relative to the spine on facing pages?

Some tutorials say to click on the spine to set it as a flipping reference point when using the scale tool, but how do I click on the spine? I can only click approximately on the spine (because my smart guides are inactive due to snap to grid that I use).
The other issue is that when I duplicate my object or group to flip it, it's off of the original, not exactly on the top of it, so that will set my flipped object or group also off.
