I have an eyelet just below grate level on my 18.5 WSM. I used to route the cooker probe on my ET-73 up to the grate and mount it with the clip but any more I just stick ii through the eyelet far enough so it doesn't fall out (and not below meat that would drip on it.) I find that the dome thermometer (I guess I'm not the only one who calls it the dome thermometer.
) reads up to 50° lower than the grate because it is close to the cold meat. Later in the smoke as the meat gets hot, the two temperatures tend to converge. I don't consider a 10° or 20° difference significant.
Were I not using a second thermometer and controlling to 250° at the dome, I'd probably get 300° at the grate which is where a lot of us like to smoke anyway. Perhaps it is even by design that holding a steady temperature at the dome results in hotter temperature early in the cook.