If you are locking the rate and the rate that is locked equals the then current index plus margin (no discount or premium) then you use that rate for your APR calculations, regardless of whether or not the index has moved at the time of closing.
If you are locking a discount or premium rate, then your APR disclosures must be based on the locked rate and then the current index plus margin at the time the loan closes (allowing for any look back period that may exist in your note).
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