i would not be getting into such detail if you do not have idea yet of the lightsource. you have started to highlight the figure, so you have SOME idea at least... it is a really, really good idea to block out areas of your image in color first, do the whole canvas in steps, rather than complete the image in sections at a time. Work out the form, the play of light and shadow, the composition before zooming in.
i find one of the toughest things about a large image is the motivation and patience required to see the project to completion.
the image is starting well, keep the updates coming!