I've found this method to practice calibration:
in the TA, or offline (change "AreanaFlagModes" to enable the accurate bombsight), you get a green + mark that points out the exact location of where the bombs will hit.
go through the calibration process, and look though the bombsight. if you've done it well, the green + will be at the center of your crosshair.
now watch it for a few seconds:
* if it moves forward (/backwards), it means your plane is accelerating (/decelerating).
* if it is just off-center, it means you didn't measure your speed correctly (wasn't steady enough).
with this method, I got best results for about 5 sec of marking. much longer than this and the result is worse.
also, you can estimate how long it takes the plane to reach steady speed (green + not moving), and the effect of opening bomb-bay-doors.
edit:
just read Anvil's post, it seems he got here 10 min before me with this method 
Bozon