I'd just insert the probe 1/3 of the depth from the top. When it hits 190 or so, go a bit deeper and compare readings. Go by lowest temp.
Also, being it's boneless and closed up, you may want run at about 240 to get above 140 internal temp in 4 hours (danger-zone).
If Your looking for lots of bark, open grate with a drip pan underneath will give the best results.