I'm now satisfied that the correct value is exactly 12.7mm (as millighost stated). This gives me a precise 90° FOV (as judged by the test below) for the default dolly cam in Poser 9. I would assume that this figure is also true for other cameras, and other versions of poser but haven't checked.

I position the default dolly cam and a point light at the origin. I created a tall thin UV-mapped vertical plane with the following texture image (well, actually a 128 wide x 2048 high extended version of it) applied to it - the image was a PNG, and the filtering was 'Crisp'. I positioned this plane so that it would lie across the left edge of a 0/0/0 (roll/pitch/yaw) render, and the right edge of a 0/0/90 render.

I then did two 2048x2048 renders with yaw=0 and yaw=90, and put them side by side. Here's a section of the result rendered using focal length 12.7298mm (the right half of every diagonal on the seam line is a few pixels lower than the left half, and this is consistent for the whole 2048 pixel height of the render):

And here's the same section rendered using 12.7mm (the left and right halfs of every diagonal match perfectly, accurate to the exact pixel, and this is true for the whole 2048 pixel height of the render):

Unless somebody can spot a flaw in my logic I think that's fairly conclusive?

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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.