The Calibration time is that time needed, to feed height over ground, speed, wind, drift etc into your norden or Lofte Bombsight manually, as it was in those old times.
At the moment we have in every bomber a supercomputer taking speed etc directly from the avionics, calculating the release point in real time and adjusting the bombsight.
I do not think, that it will be like WWIIol, were you have to take height and IAS directly from the Gauges and adjust manually the bombsight.
Here you will have to wait a few seconds.
If you drop the bombs without using the bombsight, the bombs will go the aerodynamic correct way down to earth, you just have not the slightest idea where they will hit ground by looking through the bombsight.
So if a bomber formation will drop on the sign of the pathfinder, the bombs will hit somewhere around the aimed spot, depending to th relative postion to the leading buff.
If they are higher or faster the bombs hit before the target, if they are lower or slower the bombs come shorter.
Tofri