The term “extent of growth lag” means the number of persons who would have been residents in a metropolitan city or urban county, in excess of the current population of such metropolitan city or urban county, if such metropolitan city or urban county had had a population growth rate between 1960 and the date of the most recent population count referable to the same point or period in time equal to the population growth rate for such period of all metropolitan cities. Where the boundaries for a metropolitan city or urban county used for the 1980 census have changed as a result of annexation, the current population used to compute extent of growth lag shall be adjusted by multiplying the current population by the ratio of the population based on the 1980 census within the boundaries used for the 1980 census to the population based on the 1980 census within the current boundaries.