The most sensitive HD mux in my area is the one that carries sky sports HD 1, if that is garbled or black then signal is too low, if it is fine and there are no channels showing streaky lines then the signal is just right, if you get streaky lines on your picture the signal is too strong and needs attenuating.
P.s. I wouldn't go off signal level and agc, per sae, different tuners report them differently.
No if you lose channel it is too weak, by streaks i mean like a phasing over the picture that is when the signal is to strong, i used to live right next to my green cabinet 20 odd years ago and i had to be on the lowest tap point with a 12 and a 6 (18db) attenuator on the back of the box to stop the streaky lines.
Alanp (14-06-14)