Dazzler said:
In reality, you'll only be off by a degree or two using the "nearest" method anyway - ...
In your example above (using 033 instead of interpolating to get 042 for 040) the difference would not be a "degree or two," but rather NINE degrees.
Perhaps you meant to say (I don't have Machado's book so I can't verify this) that you would use the
Correction for the nearest heading rather than interpolating. In your example, the correction for 030 was +3 degrees, and the correction for 060 was +1 degrees, so the nearest correction to 040 was the one for 030, or +3 degrees. Applying the correction for the nearest placarded heading would yield 043, while interpolating would yield 042.
Since this method agrees with your claim that the difference would only be "degree or two," I'm going to go out on a limb and say this is what you meant to say (or should have meant to say? wish you had said?).
In that case, there's a valid point to be made. Using the correction from the neareast placarded heading is certainly easier on the brain, it will usually be very close to the interpolated value, and given our ability to maintain a heading within a few degrees, it's probably as useful.* Keep in mind, though, that it's like a rule of thumb. It's a useful tool, but it won't always give you the MOST correct answer.
*It may be argued that there's no guarantee that the relationship between heading and correction is linear between each of the points placarded. The "correct" method of interpolating assumes that such a linear relationship does exist. A valid argument could be made, though, that the most ACCURATE correction might be obtained by using a method other than that described by the FAA. However, for the purposes of answering questions on tests designed by the FAA, it's prudent to use their method.
