Should I figure my cost basis for my house damage deduction based on original purchaSE price of 272,500 in 1989, or at valued price of 623,000 in 2017?

I want to get the maximum/proper deduction for Hurricane Irma damage and am not sure how to calculate fair market value of my house and property.  House cost $272,500 in 1989.  It is valued at $630,000 today, It sustained $20,000 damage. Thanks!  Pat