Voltage gain is the ratio between two voltages (Vout/Vin), so it's not measured in volts, but a dimensionless ratio (eg. 1000). You can either convert the current gain to dB, subtract 3dB and convert back to straight ratio, or convert the 3dB to a ratio and divide 1000 by that number. The book may also have taught to memorize the easy ones like 3dB, 6dB and 20dB.
The formula you've been using is fine.