The numbers from the Official record are even worse for your desired position. Hopefully either central_va or Pearidge will be along shortly to provide the source for them.
Kettel goes to length in the book explaining the sources, the use of data, and the outcomes of his computations. His sources were Customs data by port, category of goods shipped, and export value. (There are several modern economists, Adams for one, that verify his data and conclusions).
His data was reviled and attacked by other economists and newspapers of the time.
Nothing changed his facts.....more than 75% of the export value was of Southern origin. Customs data from early 1861 verified that.
The real question was how the Federal government would operate when secession permitted direct trade with the South, thus denying Federal tariff revenue. There are many records and articles on how severely crippled the US Treasury was at the time Lincoln ordered warships and military to Charleston and Pensacola to stop direct trade.
Amazing. None of them supplied any data from Commerce Department which is easy to find.
Seems as if none of them heard of the Morriil Tariff which more than doubled the tariff costs passed onto Southern consumers.
It conforms with the public school classroom narrative that the South started the war so let's talk about slavery.
No one is taught that there was an operating and stable peace until Lincoln sent the Navy to invade Pensacola and Charleston.