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yeah you are right, you have to choose lambda /4 but to be sure you have to increase as you can the distance between the structure under study and HFSS boundary to get accurate results. but i think that lambda /4 is sufficient.
Added after 2 minutes:
your right, but you have to increase as you can the distnace between the strucutr under study and the HFSS radiation to get better results. also you have to check the kind of simulation (fast , discrete) .