I use the Weber air probe and find that the pit can be 10-20 degrees F below what the controller displays for lower temp cooks; like below 300F. So I do tend to compensate and set the SF for a higher temp so my actual is closer. But I've found that for grilling temps, 350F+, that the controller vs. probe temp difference tends to become negligible. If you do start using an air probe, though, remember that nothing is instantaneous when it comes to fire. Use it to learn the tendencies of your pit, not to micromanage it. There are plenty of good explanations on here of how/why the controller temp display is "smoothed" and sometimes the "eventual" target even though an air probe would say you're not there yet. I also tend to let the pit stabilize for about 10 minutes after the SF reports it is at temp before opening the lid and putting on a cold hunk of meat. If you take a moment to understand what the SF manages to pull off just regulating airflow and fuel amount, you may find you're less irritated by the temp differences and more impressed it even manages to maintain as well as it does.
