Thanks for your reply and apologies for not replying sooner - just got back behind the computer.
I believe it is not as simple as you describe and I already gave that a try before posting my question - it is not giving me correct results. I should have been more clear on that.
Calculating the horizontal field of view is done as stated here How to calculate horizontal field of view?
I have some code that converts diagonal to vertical.
I need something similar for diagonal fov - still messin with it