You haven't said which year you did the first Roth conversion that you mentioned. Any amount that you roll over to a traditional IRA from your 401(k) in 2026 will result in the pro rata calculation being done on any Roth conversions done in 2026 (and any Roth conversions done in future years until you have no traditional IRA funds left at year-end). The relative timing of rollovers or Roth conversions done in 2026 has no effect on the calculation.
Basis in nondeductible traditional IRA contributions does not belong to any particular dollars or investments in your traditional IRAs, it belongs to your traditional IRAs in aggregate.
