Getting the true expression of the individual

When doing a portrait I seek to bring out both the fragility and the intensity of the individual, to reveal the real person. Accuracy is essential! A millimetre can make all the difference between a face that is young or aged, happy or sad, nervous or at ease with himself.
I am in heaven when I manage to capture the exact proportions and reveal the real person. 
But, like all amateurs, I must admit that I find difficult to recognise when I can go no further and decide that the drawing is finished