On or before February 15, 1977, and on February 15 of each and every year thereafter while this chapter remains in effect, the Rent Control Board shall ascertain the percentage change in the consumer price index for the period of January 21, 1975, and January 21 of the year being considered, and thereupon it shall establish a new ceiling rent which shall be based upon the percentage change in the consumer price index so ascertained during said period. No rent increase assessed against a tenant in occupancy of an apartment on the January 21 applicable shall exceed 15% of the rent then in effect.