My WSM dome therm seems to run 25 degrees LOWER than the readings I get with a digital therm placed at grate level. I get a much more consistent temp reading with a cheap Taylor therm than my Maverick ET-732 which can be squirrelly in my book. I've gotten to where I just set things to run at 250 dome reading which gives me close to my intended cooking temp of 275. It's close enough for government work and there's no complaints on the results.
I worry less about temps these days, the main thing is to have the patience to let your cooks ride out as long as necessary. Patience isn't a virtue in Que, it's a necessity.
22.5 WSM, 22.5 OTG, 1992 Smokey Joe