Guilin Sun
Ansys Employee

I would suggest to have a new post next time since the forum will be clear for one question one answer.

The example distance is arbitrary I believe, and it is not a good practice to use so many significant digits for the distance. I did not find any particular reason to use this distance. the propagation phase is about 120 deg from source to the front surface.

You can set the excitation at any distance, but "better" to be closer to the front surface. You can use 0.1um for example.  You can compare the results using different distances.