Physio-Med tried to address this problem a while ago by selling some small, reasonably cheap (compared to the professional equipment) water jars with a small power level balance. While not very accurate and not suitable for calibration purposes, they do provide the therapist with more of an indication of the dosage they are treating the patient with.
Actually the guys who came to test my unit this year used that simple bit of kit in the water jar and it worked a treat.