The correct formula, ArcSin( f / d ), is fortunately very very close to simply ( f / d ) as long as you're not doing macro. The problem is one of units, as the ratio ( f / d ) is in radians. (Note that radians, being the ratio of two lengths, are actually dimensionless units!) To translate into degrees, which is what most cameras are marked in, multiply by 180 / Pi, or about 57.3 ... actually, 60 is probably good enough, so taking the example of a 90mm (that's 0.09 meters) lens 1 meter off the ground, we get 60 * 0.09 / 1, or 5.4 degrees. The correct answer is 5.2 degrees.
Now we can see where Jack's rule comes from. For a 2 meter camera height, the formula gives a tilt of 57 * f / 2. For an extra degree of tilt, the focal length would have to increase by 2/57, or 0.0351 meters = 35.1mm. From this, we can conclude that Jack is tall.
Why yes, I was a math professor.