Family groups is always a good idea when decoying dark geese. The space between them should be one that lends itself to being a landing zone for the flying birds to join in with the crowd. I like to seperate my individual dekes by 5 or 6 feet. A good rule of thumb is "tight means flight". Birds tend to huddle closer to each other and to have more heads up when they are nervous and ready to take off. Use sentry to feeder ratio of at least 1 to 10 or 1 to 12 and give em some room. Good luck!