If I manage to screw up the courage to see about getting into mine, I know the attenuation reported by the JDSU.
A simple average could be calculated. It would be based on the sum of the attenuation calculations for the utilised frequency bands, and calculated separately for each direction. It wouldn't be very meaningful combining the attenuation figures for upstream and the downstream bands.
Alternatively, the attenuation could be apportioned according to the number of carriers in a given band and the bit loading for each carrier in that band. This metric would capture the impact of the attenuation on the quality of the connection.
The fact that the HG612 modem doesn't present an attenuation figure in its web interface could mean that Huawei hasn't got the answer either!