What cable modem signal levels are considered good ?
Downstream Power (-15dbmV to +15dbmV)
Most modems are rated from -15dB to +15dB, however it's best to have it between +8dB and -8dB. Anything less or more than that and you may have quality issues. You can remove splitters on the line if you need to raise and clean up your signal level a bit.
Upstream Power (37dbmV to 55dbmV)
The lower this number is, the better. Above 55dB will most probably cause problems, over 57dB and you probably won't be able to connect.
Signal/Noise Ratio (>30dB)
SNR is best over 30, (the higher the better, might work well with as low as 25 at times). Anything less than 25 will cause dropped connections, packet loss, slow transfers, etc.