Answer:
If your start rate is not set at the index + margin at the time the interest rate is locked between the lender and the applicant, then you would need to use the index value at the time of closing or one that was within the lookback period in your note from the closing date.
See 17(c) Basis of Disclosures and Use of Estimates- Paragraph 17(c)(1) - Comment 10 and pay close attention to 10(vi).