The limit (with infinite districts made of infinite people) is theoretically 1/4 support, in a 2 party system, with a choice made from separately decided districts. If you add another level of districts, it could be 1/8, another would be 1/16, and so on.

In practice you can’t make a district with a actually 100% support of the opposing party, and you need to leave a little room for error in the districts you plan to win. Also there aren’t an infinite number of districts lol