In the past I have paced off the horizontal distance from my stance to
the tree trunk on which my subject had been perched, did a rough
estimate of the angle from where I shot to photo to the perch or a SWAG
guess of the height of the perch, then did the math. Since I was
guessing at two of the three pieces needed to solve that triangle I was
pretty inaccurate. DISCLAIMER: This is not OCD, needed those distances
for some images.