Also, the meat will have shrunk back about 1/4 inch from the ends of the bone. Going by feel is the best test. Either the toothpick test, or grab a bone and pull on it to see if it comes apart to your liking. Or once you've done a few, you will know by how flexible the whole slab is. At 225, I wouldn't bother testing (or opening the door) until the 4.5 hour mark.