without checking to make sure I am 100% correct; your downstream power should be as close to 0 as possible but it normally works fine +/- 6. SNR should be 37+, mine is currently at 38. Upstream snr should be 47+, mine is 48 and I think up to 52 is good.
Quite annoyingly atm I have just noticed my downstream power creeping up to 4.5 and it has always been 2.something. Not good.
Have a read here if the link works:
http://www.cableforum.co.uk/board/12...al-levels.html