They fixed a certain depth as A to have a common reference when talking/writing about (120 meters maybe).
As long as bstanko6 doesn't know what depth A has been defined to, he can't know what depth the German U-Boote could reach. 'A' was defined later in war, when the Germans built better U-Boote, able to dive deeper. The allies should not get that information.
If 'A' were 80 meters, A - 80 would be surfaced. 2A would be 160 meters - correct.
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