Here's what I do, and it's pretty old school.
Open the bomb bay doors way early (like at least a half a sector from the target). Set your throttle to where you want it. Let Your airspeed
stabilize before you calibrate.
F6 to enter Bombers position
U to enter calibration mode.
Bring up the map, shift click on your target to set target altitude.
Zoom in the map and pick a point your cross hairs are on, press the Y key and hold it down while keeping your crosshairs on the point you selected for calibration. The longer you hold this the more accurate your drop will be. I'm like Larry here, I will hold it for 15 to 30 seconds..
Once calibrated you must press the U key again to get out of calibration mode before you drop or your drop will go wild.
The important thing for better accuracy is not to allow your airspeed or altitude to change once you are calibrated. Get your self set up on the heading you want to approach your target way, way early, before you calibrate. When in Calibration mode you can slew the bombsight, zoom in etc to get a lay of the land at the target. But remember, once you are calibrated only minor course corrections should be carried out and only from the bombardiers position.
Any change in altitude / airspeed will affect your drop.
As a side note, when I bomb like this I do not do it at low altitude. Low altitude leaves you no wiggle room. I bomb from 20K most times.
Also, once in the bombardiers position you are pretty much commited to the run from there. We used to call in "in the pipe" back in the old days.
Always a good idea to take a gunner with you too.
Hope some of this makes sense and helps.
cheers,
RTR