If I accept your definition of states:

" But the 1860 census shows that out of a total population of 7,568,454 in the states listed above, 3,566,298 were slaves. That means that the slaves numberd only 217,929 less than half of the total population."

" since there were more slaves than free people in that part of the country"

Less than half is not average. Nor is it more than half.