What would a Neutral NAO look like on 500mb height anomaly maps?

[weatherwiz]

I think for the most part I can distinguish the difference between a positive/negative NAO by looking at a 500mb anomaly map but I'm not really sure what a neutral NAO would look like? What I've tried to do for the most part is look at monthly NAO data computed by the CPC, Osborn, Hurrell and Hurrell's PC Based NAO data...basically if all of there numbers were within -0.5 to +0.5 for a given month I jotted that down as a neutral NAO.

Right now though what I'm trying to do is look at the NAO more in terms of which phase it was in based on a week, rather than a month.