Retirement tax questions

Thank you for the suggestion on the back door Roth contribution.  I found the back door Roth is not attractive for us as 95% or so of the funds in my traditional IRA are from deductible contributions.  So if I use the back door the $7,000 into my Roth IRA I will wind up paying regular income tax again on all but $350 of the contribution/Roth conversion.  So it seems to me the back door is really not attractive given our situation.  I would rather just use the $7,000 to pay the tax on a larger Roth conversion.  Or is there something I am not understanding about how the back door works?