It's not a hard and fast rule, I tend to look at trends more. My orp was always in the 420+ range and when that cfl bulb fell into my tank it shot to 200. I had just gotten the probe and didn't set up alerts for it, but I could have saved my tank if i had. Just the other day I cleaned the detritus out of my sump and my orp shot up 40 pts. Same happens when I put fresh carbon in. When my orp starts to trend down I know its time to change my carbon. Interestingly, when I do a WC my orp falls. I won't pretend to fully understand it, but whenever I do something to my tank there is a reaction.