I agree that this is a good reference shot and will make a lovely portrait. Besides a little more space (it might be nice to see some indication of the nearer side of the bench that she's sitting on) -- 2 areas that you may want to consider/plan ahead for are:
- her teeth (perhaps keep them soft/low contrast) and
- the way the one tendril of the girl's hair separates her head from her neck. . .you might consider pushing the hair back a bit so that the head doesn't read as disconnected (esp. if you've got other photos that show the neck more clearly or can get the girl to pose in this position again for reference)
Overall, though, very nice lighting, hand positions, expressions...
