It seems to be somewhere around 12.5-12.75mm (I'm using the dolly camera with its 'Scale' settings at 100%, in Poser 6/9/2014) - but does anybody know the exact value?

Internally Poser is built on imperial units. To get the 90degrees on a film of one inch, you will need a focal length of half an inch, or 12.7mm. Of course, all the other values suggested also sound good :-)