Do it old school, like I used to do when shooting off Estes model rockets. Go to Staples, and buy a plastic protractor. On its straight edge tape 10" of string in the middle, and hang some weight at the bottom. Walk AWAY from the tree, until the straight edge aligns with your drone, AND the string is hanging down exactly on the 45 degree mark.
Measure how far you are from the tree.....the drone is at the same height
I used to tape a toilet paper tube on the straight edge....my poor man's scope.
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Euclidian geometery rules!!