When I got my first kettle about 10 years ago, I placed a weber thermometer in the dome. I was new to smoking at that time and was pretty naive regarding dome temps and grate temps. I tried as hard as I could to keep my dome temps between 225 and 250 and ultimately that led to ribs taking ALL DAY to cook. At the time, it never dawned on me that the dome temp and grate temps were quite different. After doing some research, multiple sites kept saying you need to have a thermometer at the grate level. I finally bought a maverick digital thermometer and was amazed at the difference from the grate temp to the dome temp. The temperature difference was upwards of 75+ degrees. The light bulb finally came on as to why my ribs were taking so dang long to cook. Good luck with your cooks, hope this helps.
At this point, I still own a maverick digital thermometer (Maverick Et-732) as well as Smoke from thermoworks. They both are great products.
I also eventually installed a Tel-Tru BQ100 thermometer in the dome around the grate level. If I'm smoking something and I don't feel like getting out the digital thermometers, I still like being able to see what the temps are at food level.
I recently acquired a gently used limited edition kettle for $90 and moved the Tel-Tru thermometer from my original kettle to the new one. I can't wait to put it to work.