I programmed the MES to 215. When it reached 215 and shutoff, the Maverick was 248. The Maverick was 33° higher at this point because the Maverick probe is more sensitive than the MES sensor that is built into the back wall. That is a 33 degree difference between the smoker and the Maverick.
After shutoff, the temperature continued to rise to 249( an additional 34 degrees), the Maverick 286. That is a 37 degree difference. Here the MES digital read-out carried over another 34° and the Maverick carried over another 38° above what they were when the MES element shut off. That carryover is caused by everything being very hot (Element, walls, back, racks, water pan, etc, etc). The element shut off, but all those hot things are high enough above the air temp that they continue to raise the air temp measured by the MES & the Maverick. Some of the difference between the MES & the Maverick could be that the MES is wrong, but probably only about 15° of that. The only way you'll know how much different the two are is by getting things settled down.
Do the following with an empty smoker; If you want to smoke at 250°, start your MES set at 200°. Then see what happens. My guess would be that when the MES shuts off at 200°, the Maverick will read about 220°. Then the MES will continue to rise to maybe 220°, and the Maverick will continue to rise to maybe 255°. If that's what happens, reset your MES control to 218° (just slightly below where it stopped coasting). Now they will both drop, but the Maverick will drop faster, because like I said, it's more sensitive. Then the MES element will come back on when it gets to about 216°, but it will drop a few more degrees, and then run up to 218° before it shuts off again. You will notice that each time the MES cycles, the MES readout & the Maverick will get closer to each other's reading. When you get to 3 or 4 consecutive cycles where the spread between the two are the same, you will know how much the MES is off. That will tell you where to set your MES in the future to get the temp you want to smoke at. In other words if the difference is 20°, if you want to smoke at 250°, you will set your MES to 230°.
Note: This is assuming that you did a "Boil test" on your Maverick probe to be sure the Maverick is accurate.
So what do I set my temps to? If I want something to smoke at 250, do I set the MES to 215-220? Do I set all the temps 30-40 degrees less every time? I am not enjoying this so far.