OK I see what you mean.
It comes down to the same thing in the end.
On a modern EHSI, one method is that you place the aircraft on the cardinals (using the outsider to tell you which way to turn to be lined up) and then you enter into the EHSI config which cardinal you are on, and repeat this for the four cardinals, and it computes the correction internally.
Whereas a mag compass has to be done iteratively.
I am still looking for a method which uses GPS track, in flight. In calm air it is trivial of course, but one almost never has that. I intuitively know that a method can be developed which corrects for the wind, and I posted some stuff on it here some months ago, but nothing came of it. It would be the most accurate way to do it.
It is fairly obvious how to check a compass against a GPS on some heading (usually not a heading of your choice), by discovering the wind aloft (pretty easy with a GPS) and then flying along that heading, and then heading = GPS track.
But developing a general formula for doing the four cardinals eludes my brain...
I bet it would not be far different in principle to the triangular-track method of calibrating the ASI. Fly the four cardinals, and on each one you note the TAS, GS and the track, and plug those values into a formula and you get the actual headings flown.