29 C.F.R. § 4022.26

(a) Scope. This section shall apply to the guarantee of all benefits described in subpart A (subject to the limitations in § 4022.21) with respect to participants who are substantial owners at the termination date or who were substantial owners at any time within the 5-year period preceding that date.

(b) Phase-in formula when there have been no benefit increases. Benefits provided by a plan under which there has been no benefit increase, other than the adoption of the plan, shall be guaranteed to the extent provided in the following formula: The monthly amount computed under § 4022.22 multiplied by a fraction not to exceed 1, the numerator of which is the number of full years prior to the termination date that the substantial owner was an active participant under the plan, and the denominator of which is 30. Active participation under a plan commences at the later of the date on which the plan is adopted or becomes effective.

(c) Phase-in formula when there have been benefit increases. If there has been a benefit increase under the plan, other than the adoption of the plan, benefits provided by each such increase shall be guaranteed to the extent provided in the following formula: The amount of the guaranteeable benefit increase computed under § 4022.24 multiplied by a fraction not to exceed 1, the numerator of which is the number of full years prior to the termination date that the benefit increase was in effect and during which the substantial owner was an active participant under the plan, and the denominator of which is 30. However, in no event shall the total benefits guaranteed under all such benefit increases exceed the benefits which are guaranteed under paragraph (b) of this section with respect to a plan described therein.

(d) For the purpose of computing the benefits guaranteed under this section, in the case of a substantial owner who becomes an active participant under a plan after a benefit increase (other than the adoption of the plan) has been put into effect, the plan as it exists at the time he commences his participation shall be deemed to be the original plan with respect to him.