Thanks for the reply. So if I replay back your response, it sounds like timing of contributions during the year doesn't matter at all, and all that matters is the balance at the end of the year. If the Traditional IRA is zero all year and you do something to load it with pre-tax money (e.g. rollover) on 12/31 creating a positive balance, then you've effectively pushed yourself into having do a pro-rata calculation even if your attempt at a back door happened while the balance was zero earlier in the year.
