After a bit more empirical testing it seems to make sense now. I've attached a quit top-view sketch. So the maths is simple sin/cos trig. For pitch it'll just involve projection onto the YZ plane. And I'm not interested in Roll!

P.S. ockham - I'd never have thought of looking at the camera geometry! And you've got loads of interesting scripts there...

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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.