using 1026.17 (c) (4)...always results in an APR higher than using the actual Fed calendar figures.
Only when there are long odd days. For loans with short odd days, the APR permitted by Section 1026.17(c)(4) is lower than the exact APR.
the setting is probably Actuarial vs. US Rule
Not likely. Appendix J provides equations and examples for the actuarial method--just what a creditor will need to present a legal defense. It does not provide the details and illustrations for the US Rule method--making for an uncertain defense. Even if the loan payments are calculated by the US Rule, the safer choice for the APR calculation is always the actuarial method.