Ascertaining cost of acquisition

Dear Friends,

i have a question pertaining to ascertaining cost of acquisition of a house property. My grand father had bought a house property for Rs. 6 lacs in the Year 1986. I intend to sell it as I have inherited it. Now here it gets interesting.

For assessing stamp duty in Mumbai we have to consider ready recliner rate and add 20% to come up to built area for stamp duty purpose. Now what will be my cost of acquisition?

1) original value of Rs. 6 lacs

2) Ready reckoned rate as per 2000-01 and add 20% to come up to built up area 

or

3) only ready recliner rate as per 2000-01?