What's the pro-rata rule?
If you have any pre-tax money in a Traditional IRA when you do a backdoor conversion, the IRS treats the conversion proportionally. With 90% pre-tax IRA, 90% of the conversion is taxable. The fix: roll the pre-tax IRA into your 401(k) (rollovers don't count for pro-rata), leaving only after-tax basis behind for a clean conversion.