The export numbers we have for 1860 vary depending on source.
There are only two sources...The United States Treasury and the Department of Commerce. Both tell the same story, a distinction but not a variance.
The exports from the US that bought taxable import goods in 1859 were worth $278,902,000 at the ports of exit from the US.
Of that amount, the value of Southern goods: cotton, tobacco, rice, naval stores, sugar, molasses, hemp, cotton manufactures (all originating in the South) was worth $198,309,000 (Statistical abstract of the US Commerce Dept., 1936 edition,pgs 435-439)
.... or about 71%.
Adams uses the figure of 87% which is the above amounts, plus the value of tariffs paid on overseas purchases made with cash by Southern governments and individuals.
There were 27 Union States. By my math, they should have been paying 71% of the revenue to the Fed Gov, and the 11 southern states should have been paying 29%.
The reality was actually @$$ backwards. He is just quibbling over the most trivial aspect of these numbers and is refusing to address just how much money was lost to not just the Federal Government, but the Business interests of the North East who had Lincoln in their pocket.
It was with their backing that he won, (The North East always supports Liberal Presidents) and if he had to run without their backing, he would have lost.
This site shows US exports by category from 1860 through 1891.
The total for 1860 is $316 million, of which $191 million (60%) was cotton.
But this other site shows total 1860 imports were $362 million, of which those $191 million in cotton exports paid for 53%.
Bottom line: there's no denying that 1860 Southern cash-crops were hugely important to the US economy, however, they were not the only game in town.
And when push came to shove, Congress was soon able to find other sources of revenue to make up for import duties previously supported by lost export Southern cash-crops.