That's actually a pretty shitty metric to gauge cocaine consumption. It's being employed here since it's supposedly a great metaphor for his writing frenzy and blood always carries a certain shock value, which is tied neatly together by the blood dripping on the typewriter.

Honestly, it's pretty easy to dry out and damage your nostrils and have your nose spill blood everywhere. The stories King tells about his addiction are far more harrowing than this banal picture.