Looking at the drawings, I'd say the biggest thing value wise for you to push would be to get your darkest dark in there. Not having those dark values is causing the highlights and lighter values to not contrast and pop as much. As you're drawing and rendering, think of the form more in planes and how the light is hitting those planes. Try to think more the overall effect of the drawing and it as whole rather than focusing on the details or cool musculature. You can always spend a few minutes beforehand doing a thumbnail to plan out the drawing.

Hi Patrick, that' s nice! Perhaps I would have accentuated the lights more, which in the photo seem to me to be more extended.

well before going on shading.. i think you should sketch the fig properly... bcoz look at her face... try to draw a loomis face on it... its off proportion... quite long and head is too big compared to chin... in ur drawing the navel is more in center, but check ref.. navel is more to left side... it doesnt matter how well u shade, but if the basic gesture and structure is not good.. the fig fall apart... U can redraw the same fig... starting with proper gesture lines and structure, and then start shading.. this way it will be easy to shade also... shading on proper drawn structure is way easy!! trust me..

That's really tough ref to work from! The lighting is almost straight from the front, and a little above. So, almost everything is in the light. There's almost no core shadow—just under the jaw, a sliver on the breast, and probably an even smaller sliver on the fingers of the hand near the head. Can you explain a bit more about what you mean by just using two values? I see a lot more than 2 values. It's just a really compressed value range.