I received corrected 1099-B form with GBTC related expenses and 16 comment which says
16 This transaction represents the sale of assets from a Widely Held Fixed Investment Trust (WHFIT). The cost basis allocation factor is the value of the assets sold divided by the total net asset
value of the trust. If you know your cost of the assets sold, use that to determine your gain/loss. Otherwise, determine your cost basis by multiplying your adjusted cost basis by the cost basis
allocation factor. For example, if your adjusted basis is $1,000 and the cost basis allocation factor is 0.005 your cost basis allocated to that sale is $1,000 * 0.005 or $5. If there are subsequent
sales of trust assets, your adjusted cost basis for the next sale is $995. Sales are reported based on when and for how much the trust sold the asset. This may differ both in timing and amount
from what is distributed. There are cases where the proceeds are used to pay expenses and there is no corresponding distribution. For more information refer to regulations section 1.671-5.
When checked. the GBTC tax letter calculation amount matched the amounts under colum "1d- Proceeds &
6- Reported (G)ross or (N)etDate " column.
So the cost basis seems to be No of shares of GBTC held * Cost basis * Cost basis factor'
I am going with this logic for now unless someone gives better approach or explanation.
https://grayscale.com/wp-content/uploads/2023/01/Grayscale-BTC-Annual-Reporting-12.31.22-Final.pdf
