personally im no less than 20 and no more than 30 feet in the air. the only thing you need to know about the yardage distance is the a^2 +b^2=c^2.
x__________u in the tree
| \
| \c
a \
| \
|___b___

a= 25 ft
b= 60ft (20 yards)
c= ?lets figure it out.

25x25=625
+
60x60=3600
----------------
= 4225 (take square root of this #) = 65 feet=21.67 yards

so a 20 yard shot is 21.67 yard shot 25 ft in the tree. in other words, dont aim any different.