The determination of whether an employer is a large or small employer is subject to the following:
(1) the size of an employer that was not in existence throughout the preceding calendar year must be based on the average number of employees that the employer is reasonably expected to employ on the business days in the current calendar year;
(2) all persons treated as a single employer under 26 U.S.C. 414(b), (c), (m), or (o) must be treated as one employer; and
(3) a reference to a large or small employer includes by reference any predecessor of that employer.