You really don’t get it. It also meant British goods would be 38% less. Just do the math.
You do know today the tariff enacted by the confederate congress in February, 1861, was exactly the same as the 1857 tariff rates, right?
I guess I don't see the point you're trying to make, if there is one. In the first place there would have been a tariff on the few British goods that the Confederacy imported since one of their first acts was to adopt the pre-Morrill tariff rates. In the second place, that same tariff, which was not a uniform 10% by the way, would also apply to any goods that the Confederacy bought from the U.S. so it would increase the price of goods they used to get duty free. So where was any of this impacting the U.S.?