Roth conversions are always a proportionate mix of your basis in nondeductible traditional IRA contributions and the sum of your year-end balance in traditional IRAs, the amount converted and any other amounts distributed from your traditional IRAs that are reportable on Form 8606. Whatever amount of your basis that is not applied to your Roth conversions and other distributions is carried forward to be applied to future distributions, so you never pay taxes twice on money in your traditional IRAs.
You will always have some basis remaining in your traditional IRAs until you have a zero balance in all of your traditional IRAs at year-end.
