Calculating rise in CPI

[mcarr5]

In order to compute and forecast an estimate of the triennial rent adjustment, i need to compute the rise in the cpi (table) from the lease date (cell c3) and the present date (cell a2) or the closest publishe date existing in the table. The % rise is easy to calculate present index / opening index -1 = % rise in cpi. I update the index monthly when onthe bls publish it.