Suggestions on Lab Environment

I was wondering if anyone had some input on ideal hardware for testing BB2, BB5, Netmetal5 and other radios for mikrotik? I’m aware I need Attenuator’s, however I’m not sure how much to cut the gain, and with what kind of impedance. Does anyone have any advice they can offer me? The goal here is to hit ~-55 when two radios are wired together.