The Formula for computing indexed cost is (Index for the year of sale/ Index in the year of acquisition) x cost.
If the property purchased in 1988-89 for Rs 12.50 lakh were to be sold in 2009 -10 for Rs 1.00 Crore, indexed cost = (582/161) x 12,50,000 = Rs 45.19 lakh. And the long-term capital gains would be Rs 54.81 lacs, that is Rs 1,00,00,000 minus Rs 45.19 lakh.
The LTCG tax will be 20% of Rs.54.81 Lacs, However, you have different options to invest the capital gains to save taxes. You need to contact a local Chartered Accountant who can guide you properly.
P.S: The CII for 2009-10 is not yet notified hence, the CII of 2008-09 is considered as an Example.