For a cargo ship, go down to 40-50m/150ft, drop when the ship's masts are either side of the reflector gunsight. That was in a Spitfire, though. I'll have a go in a bomber of some description, now. B-25 I think.
EDIT: Same deal in the B-25. In a shallow dive, go down to about 50-60m and drop when the ship's masts fill the whole reflector gunsight frame. Bomb delay 2s in addition to the arming delay for both. The angle you drop at is quite important. Better if it's shallower, but too shallow and it'll bounce too far.
BTW, fruitbat, LoBiSoMeM, which recording software are you using? What settings for recording?
