You calculate the premium by adding 3 zeros to GBTC and comparing to BTC on Coinbase or any other exchange. Incidentally at the peak in 2017 GBTC got to a 100% premium. It traded at 38 when BTC was 19,000. Back then people were not as aware as how to buy Bitcoin so this was the easy way to do it. Poor fools not only lost when Bitcoin came down they had a double loss when the premium evaporated.