Hi Ed! Welcome to the club!! ;D Looks like the need to get a remote thermometer has been adequately covered, so I won't beat you up about it, too!
Remember this: BBQ is
not an "exact" science! This is not like baking a cake, with a very consistent set of ingredients, temperature and time, yielding consistent results every time. Instead, BBQ is one part technique, one part technique, and one part "feel" and experience. Unfortunately, there's one "wild card" in every smoke: The meat!! It's made by animals, and their quality control sucks sometimes! ;D As mentioned, the meat is done
when it decides it's done! That's why cook times vary, especially on the larger cuts like butts and briskets. We live and die by internal temperature! The only meat I smoke by time is ribs (not enough meat for a good internal temp), but I judge "doneness" by feel, not the clock.
A single-probe external thermometer is OK, if you don't want to go dual-probe. My opinion is that monitoring box temp is a waste of time. The smoker will "average out" to your set temp, so don't sweat it. Gotta know the meat temp, though!