Like this:
C being the distance from the stand the arrow must travel to a target
a= ground distance
b= height distance
c^2 = a^2 + b^2 ... formula to get the distance in a hypotenuse
SO.....
c^2 = 30^2 + 15^2
c^2= 900+ 225
1125= c^2
C=√1125
So your toal distance from 30 yards @ 15 ft high to target would be....
Your math is correct, but you used the wrong number. You're using yards and feet. You have to use either yards or feet. Change your 15ft to 5 yards, and it'll come out to 30.4 yards, or 3.6ft. Certainly nothing to compensate for. As others have stated, you arrow will not drop any faster from 15ft up, as it would 5ft off the ground. Gravity acts the same. As Rob said, aim for the exit hole, and don't worry about it.