(Edit - just cross-posted with your last couple of posts!)

Hmmm, that's odd. I'm definitely getting just over 12.7mm. (I'm using Poser 9 at the moment and the dolly camera - orbiting cameras have an inherent offset value which nobody seemed to be able to fully explain or give a precise value for - http://www.renderosity.com/mod/forumpro/?thread_id=2880985 )

I got my latest 12.7mm figure using this very simple empirical test setup, but that doesn't seem to be accurate enough*:

• An imported cube OBJ centred at the origin, with inward facing normals, and each face a different colour. (Imported into Poser with all checkboxes cleared so it loads as intended)

• Dolly camera positioned at the origin (DollyX/Y/Z set to zero, all Scale values set to 100%)

• One point light also at the origin (just to make things visible!)

• Render dimensions set to equal width/height.

Set the focal length to my test value.

Set roll/pitch/yaw to 0/0/0 and render. Repeat with 0/0/90, 0/0/180, and 0/90/0 as a quadruple-check.

(*The reason for this question is that I want to be able to stitch a set of six square renders into a vertical cross cubemap without any seam problems)

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The 3Dcheapskate (also available in DAZ and HiveWire3D flavours) occasionally posts sensible stuff. Usually by accident.
And it usually uses Poser 11, with units set to inches. Except when it's using Poser 6 or PP2014, or when its units are set to PNU.