The text in the upper left of the bombsight view, F6, will describe what you need to do.
1) Make MAKE MAKE MAKE sure you are at a constant speed. In my experience 2 or 3 mph error adds up at altitude and can throw your calibration out the window. Constant speed means steady AT XXX mph. At full throttle it's harder to get to that constant speed so lowering the throttle a little, the manifold pressure about 5, can give you some added correction.
2) ONLY when 1) is done go into calib mode U and hit Y key to actually calibrate.
HOLD Y down for as long as you can. 2 seconds is absolute minimum. I try for 10 minimum with 10 to 15 my usual time.
3) Note your 'calibrated speed' in bombsite view, compare this to your TAS on the Clipboard/E6B and if equal, you calibrated accurately. If not try to adjust with throttle if a mph or three off. If 5+ mph off, recalibrate , but correctly this time.
BOMB BAY DOORS add drag so you can deal with this in two ways.
A) Open them early before you start stabilizing your speed and be careful on your bomb release key.
B) Open the bay doors immediately before you release your bombs so the drag added is miniscule, very small. Remember though, even if your bombing two hangers with some distance between them you will deccelerate some and your second drop wont be as accurate as the first.
Another thing, the bombs themselves. With a formation you will drop a triangle pattern of bombs. This triangle is not always consistant and has a margin of error. If you drop when the black + is ON target in your view, the two drones will drop and hit slightly behind that +. You should try to drop when the target is in the center of your 'bomb impact' triangle.
Another thing is bomb size. IMO it's better to drop a salvo of 2 3x500lbs on a hanger instead of a salvo of 1 3x1000lb bombs. You don't carpet bomb the target but you cover it better that way and give yourself a larger margin of error.