Pencil Portrait - Beginnings

I'm taking a break from my Bargue plates, and drawing something new. I've spent around twenty hours on a portrait of my sister, working from a photograph I took in December 2011.

For the first time I'm drawing upright on an easel (rather than horizontally on a table), and on a big scale. I'm working at A1 -- whereas my Bargue plates were smaller than A4 -- and I'm using correspondingly heavier paper. At 300gsm it is twice as weighty as my Bargue sketchbook pages, and it is able to take much more aggressive erasing and re-working. This gives me confidence to lay down lines that I know can be altered later, and the large size allows me to be much more expressive when sketching out the initial form.

As the paper is so large I have drawn guidelines to separate it in to thirds to make sizing easier. I am currently refusing to use a ruler or tape-measure to precisely map the enlargement from source image to final drawing, as this feels like cheating. I instead measure relative lengths against a small paintbrush. 

Here is the sketching and rendering so far. I need to fine-tune the facial features, which are distorted at the moment, but I'm happy with how the shading on the fabric is going so far; see the image below for a close-up.

Here is the original photograph from which I am working. I desaturated it and printed it at A3, and have it taped next to the drawing to use as a sizing reference. Unfortunately the printed version has lost a lot of the tonal depth, so I use the digital version on my laptop as the source for values and detailed areas.