It’s a guess really. Here’s the thing, the meat is cooked, it’s totally safe to eat. It’s been “USDA done” and safe to eat for many hours, like 20+ hours. It just might or might not be to the texture you desire. The only way you will find that out is to experiment,
Pull the meat at X hours, keep an idea in the back of your head as to the grade and thickness, then see how you like it. Adjust the next cook up or down in time and/or temp.