Ah okay, that makes sense.
For me I usually study everything as it is so I can understand the forms and nuances for the feature. Then I'll do one or two on my own to see if I can make it fit with the others.
I think what you're doing is good practice too. It may be helpful to do one or two just like the reference to get a better idea of the plane changes and then change the lighting for a few others. On the ones you did, the the wings of the nose aren't as defined and the shapes are a little off. I think you did well with choosing the the correct values.
I just noticed this, but have you done any organic form studies? It might help to do those as a warm up since facial features are more organic.
Hope this helps!