The rules reference you back to 1026.32(b)(5) which states:
Affiliate means any company that controls, is controlled by, or is under common control with another company, as set forth in the Bank Holding Company Act of 1956 (12 U.S.C. 1841 et seq.).
The Bank Holding Company Act sets the threshold at 25%. (usually – see definition below)
Definition of Affiliate under BHC Act:
For purposes of this chapter, the term “affiliate” means any company that controls, is controlled by, or is under common control with another company.
(2) Any company has control over a bank or over any company if—
(A) the company directly or indirectly or acting through one or more other persons owns, controls, or has power to vote 25 per centum or more of any class of voting securities of the bank or company;
(B) the company controls in any manner the election of a majority of the directors or trustees of the bank or company; or
(C) the Board determines, after notice and opportunity for hearing, that the company directly or indirectly exercises a controlling influence over the management or policies of the bank or company.