It occurred to me that there may be a simple way to do this.
Pick a calm day.
Tweak your heading until your GS is minimum (if flying into wind) or maximum (if flying with tailwind).
Then the GPS track is your heading.
I tried it yesterday and it seemed to work pretty well. What helps a lot is flying in calm air because then your TAS or GS is much greater than the wind speed, and the error curve is then pretty flat (and small) over a good heading range.
I think this is just a special case of what @davids posted above.
Sounds good to me – and it is much easier to understand!
