In order to arrive at the correct calculation of any Capital Gains or Losses which result in the disposition of shares, stocks or other securities, you need to enter both the Proceeds of Disposition or Settlement Amount (T5008 - Box 21) and the Adjusted Cost Base (ACB) or Cost/Book Value (T5008 - Box 20).
If you do not have all the information provided on a T5008 slip or in your own records, you will need to acquire this information. Failure to enter the ACB or Cost will result in a Capital Gain on the entire proceeds amount.