Minor tip, instead of trying to juggle throttle, try hitting the keypad - + keys to change RPM instead of throttle. Easier to make small changes, and less likely to bump it screwing up your approach.
Also design your flight plan so that your last turn is as far from the target as possible.
This gives you time to stabilize your speed. Open doors early, use either external view, or nose gunner view to line up the plane so that it will overfly the center of the target. I do this before I calibrate, just as the target comes into view.
Then calibrate, note calibrated speed, exit calibration, pull up E6b, make the calibrated speed my ground speed.
Then zoom in to target and drop.
It will seem to happen real fast at first, but if your speed is constant going in, it won't take a long calibration.
And it won't take much to bring ground speed to calibrated speed.