Jeremy,
To convert to mV (voltage) you have to use the V = sqrt(P*R) Ohms law relationship. The system is 50 Ohm. This simplifies to
mV = 10^((dBm+46.9897)/20)
You can also derive an equation to go directly from I/Q to mV without needing to log/power the data too.
Use the 32-bit floating point values to avoid the need for an additional scalar.