Comment 37(a(10) – 3.i. states,
3. Periods not in whole years.
i. Terms of 24 months or more. For product types and features that have introductory periods or adjustment periods that do not equate to a number of whole years, if the period is a number of months that is 24 or greater and does not equate to a whole number of years, § 1026.37(a)(10) requires disclosure of the whole number of years followed by a decimal point with the remaining months rounded to two places.
None of the examples in the regulation or in the Commentary and none of the sample forms include an example that contains a balloon period that is not an exact number of years. But the Comment cited above indicates that the balloon period should be stated as 5.17 for your example when the term is not a whole number of years and the term is 24 months or more.