The only difficulty, in my experience, in the into-wind and downwind method is to accurately determine wind direction. My approach to this is to trim to a slow cruise then point the aircraft roughly into wind, then fly a series of headings at constant IAS & Alt, until I've determined the heading that gives me the lowest GPS groundspeed. Then since I'm only interested in that heading and the reciprocal, I fly on the GPS - ignoring the magnetic compass completely.
If you want the derivation of Gray's maths, and are no more in the mood to work it out yourself than the rest of us, his paper lists his Email address as
[email protected] ; presumably he wouldn't put that on a public domain paper if he objected to being asked questions about it - so if I were you, I'd do just that.
G