I think most if not all of these formulas are approximations.
There may in fact be no one formula that is "the correct" one, because a
closed form solution just doesn't exist.
Usually, these formulas have regions over which they were intended to
apply, beyond which they are known to not be as accurate. But sometimes
the formulas are reprinted without mentioning these regions, and people
get a false feeling of security that they were exact when in fact they
are not.
Try using all the formulas and comparing their results. Also try the
CAD tools at your disposal. But don't expect a CAD tool to fix
everything for you; many of them require some work on your part (because
of the G.I.G.O. problem).
In the end, nothing beats measuring it. Real board traces don't have
nice rectangular or trapezoidal cross-sections, so to some degree we are
fantasizing whenever we use the formulas.
Regards,
Andy
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