Although the instructions don't specify this, I believe line 8 is for a situation where you made a taxable ROTH IRA conversion, as opposed to the non-taxable conversion you have with a backdoor IRA. Either way, you multiply box 7 or 8 by the same factor on line 10 so that gives you the correct result on line 13 and the rest of the form.
TurboTax sometimes interprets the instructions a little differently than a literal interpretation would indicate.
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