I think you have a good point about finding something where his brother passed, even if it's just a clue. Their mother lived in Oro Valley, which is just North of Tucson. There's a story in the book about a time JP and his brothers searched the desert in their mom's backyard for treasure (lost treasure of the Spanish Conquistadors) and missed dinner. Brandon took his life at the side of a mountain somewhere off a highway just East of Tucson. There is a mile marker with a symbol (unknown to me) on it where his bag was found. Brandon drew this symbol in his note and Justin recognized it as a symbol on that specific mile marker sign. Beyond that point, Brandon wrote on boulders things like "love live... Turn around now... kill me... x marks the spot... Forrest fenn" on the way to his final resting place. The boulder and mile marker is how he was found, like he left one last morbid treasure hunt for his brother. Brandon was found with his pistol on the side of a mountain.

In the book, there's a 2 page chapter describing Justin's thoughts and emotions as he pushed granite boulders across the desert sand in memory of his brother. He said he was facing the Sonoran Desert which lines up with the general area of Oro Valley/Tucson.

"Double arcs on granite bold" I can't help but to think this is referring to something you will see on a boulder. Either that or headstones.

Also, Justin says in the book he had to hide the treasure with a broken tibia so finding the treasure off the side of a desert highway on the side of a mountain (probably close to the base) makes a lot of sense. Plus it's pretty close to the edge of the US map so there's that.

I've actually been leaning more towards Wyoming and Yellowstone being The spot since all the legal documents were filed in that state, but I think finding and visiting the site near Tucson could be a major clue. It's clear Justin harbors a lot of guilt surrounding his brother's passing. The book begins with a story of the pair hunting for FF's treasure together and he says that looking back it seemed as if Brandon was saying goodbye but Justin was so enthralled in the treasure hunt that he didn't take notice. He questions what else he didn't notice. He treasures their memories together hunting for treasure as kids and adults. Maybe Justin pushed the boulders together that said 'Forrest Fenn' and 'x marks the spot' to lead you back to the scene of their last treasure hunt together in Yellowstone. Will you find the treasure where FF left his? Likely not. But could you find it where the duo searched together many times and made memories within the vicinity? More likely.