Many of the books that talk about classic proportions of the face describe how strong the chin should be.  Here's the guideline most books mention: on the profile view, find the lowest point on the lower lip where you would place lipstick.  Draw a line vertically down from that point.  The chin should come forward to touch that line.

However, guidelines are just guidelines.  Put a ruler on your computer screen, and you'll see that, even in the after picture, her chin doesn't come forward to that theoretical line.  And we would not want to make her chin any stronger than it already is.  Often, the best surgical plan is to attempt a good improvement, and not strive for some generalized goal that might not work on everyone.