Homer said that slavery takes away half of a man's worth. So allowing 60% was slightly better than that.
There is an interesting passage in John Adams' writings where he observes a group of maybe 12 slaves doing a piece of work which 2 men could have easily done more quickly. Obviously the slaves were being clever--they had no incentive to finish the job quickly. This was probably in Washington, DC, towards the end of his term as President.
I meant to say: the 20-year delay before the importation of slaves could be prohibited...
The Confederate Constitution added a clause about the question of slavery in the territories, the key constitutional debate of the 1860 election, by explicitly stating slavery to be legally protected in the territories.