I sold a covered call on my stock, decreased my book value. How do I report that? Some calls are excised, how to adjust my book value and box 20 on T5008? Any proof need?

I always do short straddle and my self directed brokerage do not have the right number on box 20 T5008. They told me I need to calculate it adjust that myself. I know what my right book values are and have all the right number for my G&L, how do I report that under capital Gain? Do I need to provide any book value proof to CRA for me to correct my book value?