Measuring signal power in dBm as referenced to 50 Ohm impedance

[Jrbn]

When I measure 4Vrms sine signal on Spectrum Analyzer the measured power is 14.236dBm (see pic). This is due to the fact that in Multisim, dBm is referenced to a 600Ohm impedance. It says in help that "for applications in which the terminating resistor is 600 Ohm, such as in telephone lines, it is more convenient to read dBm as it is directly proportional to the power dissipation. However, in dB, you need to include the value of the resistor to find the dissipated power in the resistor". How exactly do I do that in order to get signal power in dBm as referenced to 50 Ohm impedance?