Its been a long time since I used one of those. Someone mentioned that formula in one of those "There I was" BS sessions that Old guys have. I had never heard of it before, neat trick.
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If you look at an astrocompass, you'll see that there is a way to set both your latitude AND your longitude. Getting the sun's Greenwich Hour Angle from a nautical Almanac, and then correcting this to Local Hour Angle using your current Longitude is a bit more accurate than approximating it with the "rule of thumb" you mentioned, because the almanac gives numbers corrected for seasonal variations. The almanac gives GHA and declination, and setting the astrocompass to your local latitude will align its "horizon drum" with the equator, eliminating the error due to Latitude.
BUT, the short answer is yes, that rule of thumb is correct, and works very well in Northern Canada. It works because the sun goes around the world at 15.02 degrees per hour. So you can work out how far 'round it is after noon zulu, using zulu time. From way up North, its angular distance and its geographical distance will be pretty close.
This stuff was all on the INRAT and ATPL exams when I was a student. Even though it's rather arcane ephemera now, (I can't even set the compass at work anymore, it isn't even a real compass, it's the IRU correcting to "magnetic" from a geographic database) it makes me feel a bit sad they don't teach it anymore. Being connected to the motion of the heavens through navigational knowledge felt like it was important. It felt like now that I am a pilot, they're finally dialling me into "the secrets" of how things really work, that common folk will never know.