"A number two pencil seems to work the best. Erasability is more of a factor than fineness of point."
Oh, you like to make dirt? (Sharpening pencils.) Or, you were trained in covert weapons and found out the TSA will allow you to carry sharpened #2 pencils even when you can't bring your ice pick on board?
I've found the Bic "orange" disposable mechanical pencils are really close to a really good #2 pencil--and they don't make dirt. The erasers on them are similar, but a white vinyl eraser really beats anything that comes on a pencil. They're also available at art supply stores, as "sticks" in holders that are similar to pens. At least one brand of the disposable mechanical pencils also come with the white vinyl eraser top, too.