Sous Vide and larger roast

We do a lot of whole roast, 12 -14 pounds each. We use a little different technique; we have a delta of 10 degrees between the water temperature and the desired finished internal temperature. So, if we want 140 F. IT, we set the water for 150 F. Now, like roasting in the hot air oven, the meat will over cook if left in the water too long, so the time now becomes very critical to how done the meat will be. You really need to experiment with the time unless you have a temperature probe in one of the roast. We use a special bag that has a probe inserted into the center of the roast and has a seal so the water will not enter the bag.