I think your on the right track with this approach. Using the formula that @rudyrabbit proposed A/(BxCxD) doesn't seem to make sense because you end up with a value that is in BTC and you need a value that is in dollars.
So you really need to just multiply the total $ you invested as into GBTC and multiply that by the Cost Basis Factor in the 1099-B to obtain your Cost Basis that And if you are making multiple purchases of GBTC, then you'll need to calculate your total amount of money invested (eg, Basis) for each transaction by looking up the share price of GBTC.
