Value, in the artists' world, refers to the depths of light and dark one achieves. It can be established on a scale of 1 to 10 with the lightest starting at 1 and the darkest being a 10. Hence, you would achieve a halftone at a range of 5 or in other terms 50%, which would be a grey tone area in your charcoal drawing. To improve and practise your toning skills, start your toning from the centre of the image. Keep your darkest shading within the range of 6 to 7 and the lightest at 3 or 4. Work your shading outwards, from light to the darkest shadows. Practice makes perfect, and dabbling in different value ranges will give you an understanding of their effect.