Dear Dale,
As I know, There is no specific values ,but you can consider the following ranges to troubleshoot the dsl issues .
For Noise Margin: (the higher this value, the better)
8-13 Average
14-22 Very Good
23-28 Excellent
For Attenuation: (the lower this value, the better)
20-30 Excellent
30-40 Very Good
40-60 Average
Also incresing the Increasing the transmit power levels will also improve the noise margin but at the cost of interfering with other services in the same cable.
Regards,
Haris