Justice Breyer does the math.

Yesterday, the Supreme Court heard argument in Bartlett v. Strickland, an important voting rights case:

Christopher G. Browning Jr., North Carolina’s solicitor general, defended the decision of officials there to violate a state law in order to create a district that included about 39 percent of the black voting-age population, saying the Voting Rights Act required the creation of the district to prevent the dilution of the minority group’s ability to elect a representative of its choice.

The fact that the district did not include a majority of black voters was a virtue, Mr. Browning said. True, he said, minority voters would be able to elect a representative of their choice only with the aid of voters from other groups. “Coalition districts help us in reaching the point where race will no longer matter,” Mr. Browning said....

Several justices seemed [say or suggest] that a 50 percent requirement had the usual costs and benefits of what lawyers call “bright line rules.” They are easy to apply, but they can be arbitrary and inflexible....

Justice Stephen G. Breyer proposed [a] number, one tied to the amount of crossover voting from whites needed to elect the minority group’s preferred candidate. “There’s a kind of natural stopping place,” he said. “When I worked out the numbers, it seemed that natural stopping place fell around 42-43 percent.”