Doodles and Finished Pieces

[Chutzpar]

It's mostly the shape of the breasts rather than the size
I mean, not saying it's impossible, but they're of a shape that's a lot less common than TV would lead you to believe :V
If she's had a reduction then I take it back, though! In a reduction a surgeon will usually aim for a pretty idealised shape, for obvious reasons. Pardon my assumption then c:

Also yeah, getting proportions right when you can't see the head very well is a huge pain, since there's not a whole lot else that's particularly practical to use as a measurement

Charcoal isn't super good for contours; I tend to use a fairly hard pencil or a pen for linework personally, since they tend to be more precise. Charcoal's a pretty fast medium that doesn't lend itself very well to accuracy!
You could try using compressed charcoal or a charcoal pencil if you still want it to be fairly movable but want greater precision c:

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OH
and if you don't already have one, invest in a putty eraser. They're a lifesaver when you're working with charcoal or chalk, seriously.