Pork Butt Question

I'm doing a pork butt and want to cook it at about 225.  I'm going to use a Maverick to monitor the temp, but should I monitor it at the grate or should I attach the pit probe to the dome.   I frequently have about a 20 - 30 degree difference between the dome and the grate and over the last few longer cooks, the difference didn't disappear.

In the past, I always set the probe at the grate and kept it at 225 but now I'm wondering whether I was really cooking at 250 - 260.