True quarantine is quite difficult. Not many backyard peeps do it ,,, and those that do,,, are not really achieving the results a true quarantine would.
I believe the distance should be at least 400 feet.
You would also need to change clothing, and footwear when going between one, and the other.
Another note about Mareks. That stuff does not go away with quarantine. If Mareks is in your grounds,,, it will be there for a very loooooong time, even after you remove all your chickens.
My view about introducing new birds to a new flock.
Get your chickens from a known healthy flock. I know,,, Try Your Best. Then introduce them separated by a fence. A chicken wire barrier works well. Then after about a week, let them mingle together supervised.
If there is vicious bullying, then consider keeping separate a while longer. There will be pecking order spats,,, so be aware of those. As long as there is no REAL harm/injury done flock should integrate well.
WISHING YOU BEST,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,