That is pretty much correct. For the average whitetail stand hunter, the minor angles encountered will not be a problem using a rangefinder that does not account for them. At 20' up a tree, you will be only "off" about 1 yard at 20 yards.
However, if you may take extreme shot angles in very rough terrain (over cliffs, river banks etc), you have to account for angle. At a 30 degree angle from horizontal, for every 1 yard 'line of sight' the target is away from you, the actual horizontal distance is 0.87 yard (at 30 yards 'line of sight', this means the horizontal distance is actually 26 yards).
At a 45 degree angle from horizontal, the actual horizontal distance is only 0.71 yard for every 1 yard 'line of sight'.
Hope that helps!
Last edited by JoeRE; 03-02-2010 at 09:34 AM.
Reason: Maths right now :)