I agree with your interpretation. Your contract should say what your look back period is.
1026.20(c)(2): Timing and content. Except as otherwise provided in paragraph (c)(2) of this section, the disclosures required by this paragraph (c) shall be provided to consumers at least 60, but no more than 120, days before the first payment at the adjusted level is due. The disclosures shall be provided to consumers at least 25, but no more than 120, days before the first payment at the adjusted level is due for ARMs with uniformly scheduled interest rate adjustments occurring every 60 days or more frequently and for ARMs originated prior to January 10, 2015 in which the loan contract requires the adjusted interest rate and payment to be calculated based on the index figure available as of a date that is less than 45 days prior to the adjustment date. The disclosures shall be provided to consumers as soon as practicable, but not less than 25 days before the first payment at the adjusted level is due, for the first adjustment to an ARM if it occurs within 60 days of consummation and the new interest rate disclosed at consummation pursuant to § 1026.20(d) was an estimate.