Look over the whole drawing and compare it to your subject, Sometimes a little 'artistic licence' might be used to emphasize shadows and improve the form. This is an informal sketch, not a photo-realist drawing. So I haven't tried to draw all the spots or create a perfectly smooth surface. Pencil-marks are allowed to show and can make the drawing more interesting than if it was perfectly even. My original drawing has a bit more variation in tone than the scan shows. It's not perfect, but I think it's not bad for a quick sketch, so I'll call it finished before I start to mess about with it too much!