I'm going to chime in on the 100% sure mindset. I much prefer the "call what you see" attitude because oftentimes we aren't 100% sure when making a call, especially in two man. As BU, the play at first base is my responsibility. Sure, PU can keep an eye on it and gather information, but it's my job as BU to determine whether the batter runner is safe or out. Being inside the diamond, you often aren't going to see the actual space between F3's foot and the base. However, you can tell if he had to make a longer stretch than normal and whether or not his back foot got dragged off the base. I still need to be confident that F3 came off of first base to call "safe, off the bag", but I don't need to necessarily see the gap to know that he came off the bag.