You want your NAs organized by context (like calls to make, things to do on your work computer, things to do when you're in the car running errands, etc.) so you can maximize what you're able to get done in any of those contexts. Any NA tracking scheme will work if it lets you do that. Different people's contexts look different, so some people have interesting variants, like a list of five-minute NAs for when they're between two calls. Give some thought to the kinds of contexts you find yourself in in which you could do some useful work.

I create to-do lists that last for a day or two, which are a subset of my defined NAs. That's not strictly canonical GTD, but it works well for me, and it seems I've heard from a lot of folks who do something similar.

(Actually, I've seen David Allen post along the lines that anything that lets you get more stuff done with less brain damage is "canonical" in his view.)