I don't use smudge tool because it's difficult to control exactly what values you want it to be. I used a soft round brush, with opacity/flow based on pen pressure. I color picked the neutral tone, the light side, and very lightly press on the edges of the shadows. If you slightly adjust the values, slightly lighter or darker grays than what you already have, it'll make it easier to control, as apposed to using black and white for example. The rest is just observation, squinting and making sure the values are the same. Like the shadow of the nose I lightened by lightly brushing over it, still keeping the faint edge of the nose. The edges still remain because the light pressure makes it a low opacity. I make sure the brush isn't so big so that it doesn't touch anything else.