A Roth conversion of $6,000 with $100 remaining in traditional IRAs at year end would be $6,000 * $6,000 / ( $6,000 + 100) = $5,902 (rounded) nontaxable and $98 taxable. The $100 remaining in the traditional IRA would include $98 of basis. (In this scenario it would make sense to just convert the entire $6,100 resulting in $6,000 nontaxable and $100 taxable.)
