This is a rough guide only, but if you put your elbow against the nose of your saddle, your fingertips should just reach to the top of your handlebar, i.e., the distance between the nose of the saddle and the back of the center section of the drop handlebar should be about a cubit. All of my bikes are slightly longer than this, which works OK, but I would not want to go any longer in the stem or top tube. (My situation is also complicated by my right radius now being a full 2cm shorter than my left, after a fracture doubled my developmental difference.)