When figuring that a horse is 2 wide, or 3 wide, what is the distance between horses (center to center) that is used?

In looking at videos, it looks like a horse that is 2 wide is about 4 feet, center to center, from the horse on the rail, and that a horse that is 3 wide is about 8 feet, center to center, from the rail horse, etc.

Any info on this would be appreciated.

Thank you.