By saying the payments are "variable", do you mean they will only change if the index changes? If that's the only issue, base your APR on the interest payments you calculate at the index value in effect at the time the disclosures are being prepared.
On the other hand, if the schedule is something like "principal payments of $100 per month plus interest at xx%", then every payment is different & you have to load them all.
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...gone fishing.