Gerard,
Borislav has shown you how to do what you are asking. I would just comment that if I were the user and I could not click right where I wanted to edit I would be annoyed. If, for example, the value was 124.53 and I wanted to make it 125.53 I would click right before the 4 so I could do a Del and type 5 and be done with it. Your approach to handling the mouse would be frustrating to me in that situation.