Right, no I haven't forgotten about this thread...
So I looked at the formula you derived, and I'm pretty certain there is an error in there, TBH I'm not sure how you derived it, but thanks for trying to help me out. Basically the problem is the vertical FOV becomes smaller with an aspect ratio closer to 1:1 rather than larger as it should.
I spent an hour or so last night trying to figure it out from the basics, thanks for the wikipedia link BTW, and this is the formula I've come up with. I guess it could be simplified, but I can't see how..
V = 2 * arctan (0.5B * (tan 0.5H / 0.5A))
Where:
V = Vertical FOV (degrees)
H = Horizontal FOV
A = Aspect width (e.g. for 16:10, 16)
B = Aspect height
I haven't derived one for the horizontal calculation, since I'm not as interested in that.
Using the above formula with a horizontal FOV of 90 degrees, and an aspect ratio of 16:10, it gives me 64.01076642 vertical degrees.