As I indicated in my last comment, it depends on if you are calculating a gain or loss.

Gain is based on the Cost.  The cost starts the ownership period, so you do the percentage based on the entire ownership period (the date of purchase until the sale).

If there isn't a gain, then you determine if there is a loss.  Loss is usually based on FMV.  FMV starts the business period, so you calculate the percentage based on the date it was converted to business (the date of the FMV until the sale).