I reckon the knock read out is differant for every car, more of a guide than a measuring tool
Mine is all over the shop sometimes, I think my sensor is stuffed, either that or my exhaust or something hits the bottom of the car now and then and it picks that up. I remember when I was getting it tuned for a street meet ages ago, we pulled almost 6 degrees timing becasue the knock light was flashing, dropped 50rwkw, and it would still register knock. Coming back up slowly on the timing, we found it could take another 4 degree's (10 degrees above the minus 6) before it did knock, and let out a bit of a puff of unburnt fuel out the back, and the knock value stayed the same.
So I reckon its more of a guide than a measuring device, either that or mine is rooted!
That said, it has been quite handy on occasions, can tell the differance in knock levels when its got shit fuel in it, but there are greater things at play I think with my car....... something is making noise and it aint detonation!