The "probing like butter" is really easy to distinguish from "not done yet". Usually, somewhere between 190* and 210* the thermometer probe will slip right into the meat. It is a dramatically different feeling than in raw or not done yet. If you were in a cartoon, a big lightbulb would go on over your head. Trust the "probes like butter" advice, you can "get it" on your next brisket.
18.5 WSM, Green Weber Genesis320, Cast iron firepit(w/grill), and a Copper Weber 22" OTS