While your math is correct, that is not how the IRS calculates tax due. They use a tax table and it goes in $50 increments of taxable income. I have included a link to the table below, but you will find that $68,250 - $68,300 for married filing jointly results in $9,314 in tax due. Also, note that this table is not used in every case - when you have income that is taxed at different rates, such as capital gains, there is a different worksheet.
https://www.irs.gov/pub/irs-pdf/i1040tt.pdf
