Hi Monika,
If you are using one DP_QPSK for each WDM channel, then the power per channel power will be the same as that determined for each DP_QPSK. Otherwise, it depends on how you use optical sources in your system.
When I want to analyze a system at a constant BER, I use “iteration” to repeat the simulation, so that each time I simulate, I have a better chance of getting the BER I intend to get from one of the iterations. This is merely my experience.