• There was a set of backdrops on Renderosity with such guides on, called "Divine Poser" (no longer sold, though).

• Krios posted a Poser hack on a Smith Micro forum thread called "The Rule of Thirds", and about two thirds of the way down he links to a .zip file on DeviantArt (still live). It can still be found via Google. From his readme...

The Rule of Thirds by Krios Menoti - August 31st, 2017. This is a simple geometry (obj) that replaces the standard Poser Depth of Field guide with a version that can be used to figure out the "sweet spots" of the Rule of Thirds.

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Learn the Secrets of Poser 11 and Line-art Filters.