TJ is very spot on with his evaluation. I have a decent amount of experience with the Aeroforce tech gauge (I bought one in the first ever group buy, they were originally produced for the grand prix)...Knock retard is just a measurement that is showing how many degrees of timing is being pulled because of detonation. As far as I know, there isn't a real measurement for knock...the knock sensors just know when they see a small ping or a big ping.
The level of ideal knock retard is always going to be 0 (for safety and longevity of the engine at least...), some of the grand prix guys made their fastest times with 3-5* of knock retard (including myself), but I feel that losing a tenth in the quarter mile is worth it because you are making the engine more reliable.
Hope that helps just a little bit...gotta go run to the bank to go get a cashiers check for a certain car...but if you have any more questions, just ask..and I'll try to answer if I can.
edit - oh, I forgot to mention this.
Knock Retard in a certain level can't be labeled as dangerous or safe per se, because if you have 3* of knock retard on stock timing advance and boost levels, there isn't as much danger as if you've added say 5 degrees of timing advance, and 4 more psi of boost. This is because timing advance adds extra cylinder pressure, and boost also adds more cylinder pressure, as well as higher compression ratio (this is why lower compression can run more boost)...but if you're running a lot of boost and timing, and get a hard ping, it's going to do a lot more damage to the piston than a ping of the same level, but with less cylinder pressure. It's like hitting the piston with a 30 pound sledge hammer compared to a 20 pound sledge hammer (those numbers were all completely made up by the way, just hypothetical.)
