well you could use the formula, but then Zeiss SF:s (8x and 10x) would have 69° AFOV, so they would still be ahead,
but apparently neither zeiss or swaro use that formula,
all the figures are from the manufacturer themselves,
probably more realistic values:
http://aa.swarovskioptik.com/nature/el-c21011001/el-8p5x42-p5222897
And if you compare them IRL you can
spot the difference, which was my point.
Zeiss SF have the bigger AFOV (and bigger FOV obviously),
and it is noticeable.
People usually don't like a tubelike view
you get with a smaller AFOV,
personally I think 60° - 70° is acceptable,
with the ideal somewhere in the middle.
higher AFOV gives a more immersive view,
if the ER is long enough,
otherwise you may not be able to see the whole field
without peeking around corners/moving your head
but Zeiss have done a great job with the eye piece in SF,
will be interesting to see if Swaro will follow this path
or not.