1: this webiste https://optics.ansys.com/hc/en-us/articles/360034914813-Integrating-power-in-far-field-projections is correct for calculation the power in the farfield.
2: usually you do not need to place the monitor far away to be twice the wavelength, except you simulate larger volume. For better accuracy the monitor can be closer in general
3: for a dipole source, if you only use one planar monitor instead of a box, it is understandable that the further away the monitor, the smaller the power it records, as it cannot pick up the power with larger radiaiton angles. Thus, to get better result, the monitor should be closer, and the simulaiton area along the monitor should be large enough and the monitor edge should have almost zero intensity. It can be very challenge when a dipole radiates in -90 to 90 deg, eg, the strongest radiation is along the monitor surface. This is related to dipole polarization.
You are navigating away from the AIS Discovery experience