I'd have not the least idea how to determine if it's 1/100, 1/200, 1/1000, so maybe you guys can educate me on that part.
It is mostly the question.
As a beginer, I was climbing in remote area in Quebec. My biggest fear was to be trap high above my last pro with nothing else to do than jumping. So, I began to try to evaluate when I can go and why I should bail. Bailing is some times more painfull than risking as you remember for many year that you should have try... Fortunately, I began at a low level, 5.7 in remote area. It is close to a route like recompense. When I trained, going to the top was not an objective any more. But learning the limit of a movement was the goal. In ice, I trained pure technique (just crampon, crampon and one ice axe, pidgeon hole...etc) and at the end of the day, I did top rope very fast to understand what was my mistake. When I wasn't in overtraining, my technique was better than today. As I lead, and as Sneoh explain on overestimating the risk, my evaluation of risk change. There is real risk and false one. Someone soloing with 100% chance of doing the move is less careless than a newbye going out of the gym. But try to tell to a 19 years old guy who climb 5.11 in a gym that he didn't have the experience of a 5.7 climber. Try to explain that, as you know more technique, you have a better idea of how to evaluate the other climber and, by imitation of good climber, you can improve in safety.
As for the 1/100, 1/1000 I can not explain it for other as I don't know the physical condition and difference of reach of the other. It is understainable for people with some academic background. I always try to climb with people of different level. Some times, beginer are more natural than old one. Actually, I like to climb with very experience climber, Base, Steeve Arsenault, Joe Cote, Georges Hurley, etc. There is a very good description of a technical climber on the forum, Bouchard, climbing repentence this winter. All them climb so naturally with good technique and they are so generous to show you your mistake ( as when you have hard time to do a move for fifteen minutes and he came after with the question: could I? And do the move very easily). Or climbing the book of sol... in a 5.10+ variation to see if you read the rock at 5.9 or follow what the leader did. Climbing with your girl friend some week end and finally going to the big one with a good friend. In all that, it is not a solution that someone told you, but an accumulation of good and bad experiences who bring you to the evaluation of a movement and it seriousness.
So, I don't know for you at to calculate the risk, but I know for me. As much I climb in different area, better is my evaluation.