All assessments shall be determined by proper classification and rating of the risks which an assessment association may assume, so that every member may be assessed in a proper proportion to his risk. The method of estimating the pro rata amount of each member’s liability for losses and his share of the expenses shall be fixed in the bylaws, which may provide for assessments for not more than one year in advance in accordance with the amount estimated by the board of directors to be necessary to meet the losses and expenses of the association for such period; Provided, any assessment association maintaining the same reserves, surplus, and contingency funds as are required of a stock or mutual company licensed to transact the same kind or kinds of business, may make advance assessments for periods longer than one year at the discretion of its board of directors.