Scot,
As you learned something from your mistake, that alone should constitute a successful cooking lesson. Folks who cook the most are bound to make the most mistakes.
I refer to the OE dome mounted Weber thermometer as a hood ornament on a nice car. Looks snazzy and racy, but that's just about all it's worth in terms of actual performance.
Of course you will get tons of tips and advice from posts on this board. Some are good and some not so good, so filter it all carefully. Most advice comes from personal experiences so some value can be extracted from most.
Dave offers solid advice to use a quality aftermarket thermometer and to position the sensing probe close to where the meat will be on the grate. I use and recommend a Maverick ET-732 remote system. Oddly, the internal temps of a WSM can vary from place to place since convection is moving heat all around inside. Measuring near the meat is the real deal since thats what your meat cooks at....not the dome temps. Hanging the probe thru the vent hole works well if you are not a frequent "peeker". BTW...If you're lookin' you ain't cookin' goes the saying. I personally installed eyelets thru the walls so the probes could go inside the WSM w/o cluttering vent holes. Again, personal preference.
I can assure you will love your WSM as soon you and and "it" become best friends....and what I mean is that you are famailiar with the operations and factors influencing your cooking successes.