First of all you're need to make sure that your receiver has 50 Ohm impedance at specified frequency.
Very often it is not 50 Ohm. You can check it with vector antenna anlyzer.
The second, there are different volts: Vrms, Vpk, Vpk-pk.
So, if your receiver input has Z = 50 + j0 [Ohm], and you're talking about Vrms, then your calculations are valid.
For the reference, with Z = 50 + j0 Ohm:
0 dBm = 0.001 W = 0.2236 Vrms = 0.3162 Vpk = 0.6325 Vpk-pk = 106.9897 dBμV PD = 113.0103 dBμV EMF = S9 + 73 dB HF = S9 + 93 VHF
If your generator is marked for peak-to-peak at 50 Ohm load, then power can be calculated with
P = (Vpk-pk/2)^2 / (2 * R)
For Vpk (peak amplitude):
P = Vpk^2 / (2 * R)
Please note that Chinese DDS such as FY6600 shows invalid voltage. As I know it shows Vpk for open output (with no load at all). It means that you don't know what voltage will be with the load, you're needs to measure it. But if you know output impedance of your DDS is 50 Ohm, then you can just divide the value which is shown by two.
As I know FY6600 has 50 Ohm output, but you're need to check it. In order check, do the following:
1) Select 50 Hz 1 Vpk (1 Vpk = 2 Vpk-pk) sine on your generator, but do not connect anything on the output, leave output connector open
2) Measure RMS voltage on the output connector with TrueRMS multimeter.
3) Connect 50 Ohm resistor on the output connector and measure RMS voltage with TrueRMS multimeter.
If your gen has 50 Ohm output, the value measured on the step 2 will be exact 2 times higher that the value measured on the step 3.
If your multimeter shows 0.7071 Vrms on the step 3, then your generator shows correct voltage.
If your multimeter shows 0.3536 Vrms on the step 3, then your generator shows incorrect voltage (this is Chinese volts). It shows voltage for open output. So, you're need to measure actual value with connected load. If your gen has 50 Ohm output, you can just setup twice higher value on the gen in order to get required voltage.