For the downstream you are looking for power levels as close to 0 dBmV as possible within a range of about -6 to +10 and a S/N ratio above 35dB (officially I think the limit is 33 but it'll be really flaky if that low). For the upstream you want four bonded channels each with a power level somewhere between about 35 dBmV and the high 40's, you'll have issues if it creeps much higher than 51 or so. The other thing to watch out for is the modulation for the upstream channels, most areas on VM's network now support 64 QAM (there may be some which still don't so may still be on 16 QAM), if the line is 'noisy' then this can drop to 32 or 16 QAM.
However remember that all these are only really guidelines, in reality there are differences in the tolerances of the components in the Hubs and in the CMTSs, also a downstream power level of, say, -7 might well be OK if at the same time the S/N ratio for that channel is high at 40 dBmV. There's no real 'if the value gets to this critical value then you will have issues'