You said: “Your data confirms one very important element of my overall argument: total exports for 1860 were $400 million.”
That number is the total, but includes specie of undetermined source and international reexports from other countries being transshipped through US ports.
So, if you want to make some point about the percentage of Southern produced goods measured against the total export value of 1860, the you will have to use the number $316,000,000. (See page 885, column 5.)
No, your own link shows total 1860 exports as $400 million, including specie.
And that is the number, not $316 million, which paid for import tariff revenues.
Further, strictly Deep South exports were $192 million for cotton, not the much larger number which includes Union produced goods.
Bottom line: it's totally fair to say that 3 million Deep South slaves produced 50% of US export revenue, but not the higher numbers 75% or 87%.
And we know this for certain because when cotton came out of the mix in 1861, US tariff revenues fell by only 26% and soon strongly recovered.
So Deep South produced products were not as important as some then & now claim.