Mont. Code Ann. § 19-3-2114

For an employee who elects to become a member of the plan, the board shall transfer from the defined benefit plan to the member's retirement account an amount equal to the amount that would have been allocated to the member's account pursuant to 19-3-2117 had the employee become a plan member on the employee's hire date, plus compounded annual interest equal to the assumed rate of return on investments adopted by the board as of the effective date of the transfer.