In 1860, Slavery was illegal in most northern states. I think you'd have to say that the American South was the realistic place to draw the boundary line. According to a teaching company course, in Mississippi and South Carolina the slave population was way over 50%.

I am also skeptical about there being only 5 countries with slave populations over 20%. The British and Dutch Caribbean and Haiti had slave populations much higher than that, judging from their current Black populations. And so many other countries kept slaves, it's hard to believe there aren't others who hit the 20% mark.