Q: How far apart do decoys need to be to be fully effective, and how deep does the horseshoe pattern need to be as well?
A: This is not an exact science, but as a rule of thumb, pigeons tend to feed closer together in the winter than they do at other times of the year, therefore your pattern needs to reflect this.
The only reason we decoyers try to shape our patterns is to lure pigeons into the easiest shooting position, rather than trying to simulate a feeding flock. Pigeons DO NOT feed in any kind of pattern!
As long as there is enough room for incoming pigeons to attempt to land safely, your pattern will be effective.