It is well stated in the bombing rules that no measurements may be made to assess the success of a bomb release.
So what follows should be considered as a visual approximation that an experienced pilot would use before dropping his destructive load. By the way, at that time engineers were beginning to develop more and more precise bomb aiming and dropping systems.
But this is not a hidden cheating operation!
It is not a question of putting cards between the plane and its target to measure their distance.
The goal is to visually assess the distance between the plane and its target using the length of the playing cards.
On the tables (picture below), the space between the horizontal lines represents the length of a card.
It shows the number of cards (taken lengthwise) separating the red dot on the target card to be bombed and the base of the plane when the bombing is announced and before performing the first maneuver (straight/turn or stall) of the bombing sequence.
To be sure to hit the bull's-eye (red dot covered by the bomb card), you must announce the bombing when the front of the plane's base is in the light gray zone.
Further forward of this zone, it is too late, the bomb will pass the red point.
Behind this zone, the plane is too far, the bomb will fall in front of the red point.
The destruction of the target will only be partial if the red dot is not covered by the bomb card.
If the first maneuver is a turn (then, a straight will be placed in front of the plane to place the bomb, see basic rule), the number of separation cards will be the same as long as we "measure" from the middle of the front edge of the base (where the small black line is in the middle of the shooting cone).
For example, a K deck plane at altitude 1, may announce the bombardment before performing a straight line (or a turn) if the base is at a distance from its target (red point) between 3 cards farthest and 2 cards + 1 centimeter closest.
If there are only 2 cards away, the bomb may fall behind the red point (half damaged if the red point is not covered but the target card is partially covered).
So if there are 2 and a half cards, you are sure to hit the target !
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