What is considered "ideal" nostril show from the front, for a woman?

The amount of nostril show should be 2-3 mm from the slide and 2 mm from the front. That depends on the angle of the nose as well.  Digital computer imaging would be helpful to understand what can be accomplished with your nose upon your facial features. For many examples of columellar show repair, please see the link and video below

Excellent question.  There is no accepted "millimeter" amount of nostril show that is considered ideal on the frontal view.  What is often discussed is a gentle "sea gull-in-flight contour" of the nostrils and columella of the nose (the flesh between the nostrils).  This contour and the amount of nostril that you see on frontal view can be affected during rhinoplasty by manipulations of the nasal tip cartilages.One thing to think about is head position when looking at your own frontal view in a mirror.  If you tilt your head back, your nostrils will show more and vice versa.  Surgeons place patients in the Frankfort Horizontal when analyzing the nose.  That means that a line drawn from the lower orbital bone to the top of the ear canal is horizontal.  Please read more about "Prerhinoplasty Facial Analysis" in an article I wrote for Medscape.