I do that a little difference, once at altitude I cut throttle back to 80 percent, on a b17 this is manofold 40 setting. this give u a true air speed of between 200 and 210. A slower drop speed give you better accurracy. also one thing else if you miss right or left and the cross hairs were on the target, then durning calibration you had the joystick with just a little roll added into the process. Do not move the joystick either to the right or left when calibrating.
In other words practice makes you better
DarkHawk