HFSS far field calculation distance?

Hello ,

I would like to understand at what distance HFSS computes the far field amplitude and phase? I want to simulate the e field phase at a given distance ( but in the far field zone) while moving the horn generating this field +/- from the reference plane.
Is it OK to move the horn and consider the far field measurement location stays the same? Should I move the radiating box as well?

tnx

Dom