A judge who renders part-time service after January 1, 1990, shall receive a reduced retirement allowance. The reduction shall be based upon the relationship between the actual service rendered by the judge, including service rendered by reason of sitting on assignment, and a full-time judge’s service during the period from January 1, 1990, until the date of retirement. Computations under this section and subdivision (a) of Section 75076 shall consider the salary payable to the judge of a municipal or justice court to be equal to 91.3225 percent of the salary of a superior court judge. For purposes of qualifying for retirement, part-time service shall be the equivalent of full-time service.