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* 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.

*also available in ShareCG, DAZ, and HiveWire3D flavours (the DeviantArt and CGBytes flavour have been discontinued).