I don't think these stove top cat probes are very accurate due to the radiant heat coming off the stove top so when I reload on top of coals I still wait at least 10 minutes to get the wood good and charred. By that time I can be well over 500 or even close to 1000. Heck, most of the time my probe is 600-700 when it's time to reload but I won't just throw the wood in there and engage the cat because I know it's probably off some. A cold stove is a different matter and the probe temps are probably a little more realistic. After awhile you get the feel for a cat stove and can tell when to engage more my looking at the fire and just giving it a little time.
I usually only pay attention to my probe when it's a cold stove, if it's already active I pretty much follow the same method as you.