The board of supervisors in any county, by a majority vote, may enact an ordinance providing that the maximum annual allowance payable to a member pursuant to Section 31727.4 who is totally disabled shall be increased to 60, 70, 80, or 90 percent of the member’s final compensation, as determined by the board, on the operative date of such ordinance.

For purposes of this section, “totally disabled” means inability to perform substantial employment and the presumptions contained in Section 4662 of the Labor Code shall also be applied to the determination of total disability.