Mo. Rev. Stat. § 86.127

The annuity savings fund shall be the fund in which shall be accumulated contributions from the compensation of the members to provide for their annuities. Upon the basis of such tables as the board of trustees shall adopt, and regular interest, the actuary of the retirement system shall determine for each member the proportion of compensation which when deducted from each payment of his prospective annual earnable compensation prior to his attainment of age sixty and accumulated at regular interest until his attainment of such age shall be computed to provide at that time an annuity equal to the pension to which he will be entitled at that age on account of his membership service. Such proportion of compensation shall be computed to remain constant.