Adjust the gain of the tail but turning the right pot on the gyro board. The left pot does something else so leave it alone. Turning the right pot a hairs-width clockwise will increase the tail rotor sensitivity (thereby making the heli turn more counter-clockwise when viewing from above). Turning the right pot a hairs-width counter-clockwise will decrease the tail rotor sensitivity (thereby making the heli turn more clockwise when viewing from above).
Just play around with it until you get it right. Remember, you need the hand of a surgeon to get it just right, so be patient and careful!