A plan under which the districts establishing it agree to pay to employees who become entitled to retirement salaries within a specified period, not exceeding 15 years, after the establishment of the plan, a specified sum that, during the life expectancy of the employees, will be approximately equal, in the aggregate, to the aggregate difference, during the life expectancy, between the maximum salary paid to employees in the respective classes of the retiring employees, and the salaries paid to beginning employees in the classes, shall be construed to comply with the provisions of this chapter requiring the plan to be in accordance with sound business practices and recognized actuarial methods.