The re-enforcing is not completely ahistorical. In the actual operation Zaunkönig where these acoustics where first used the attacked convoy was strengthened by combining another convoy and a support group:
https://en.wikipedia.org/wiki/Convoys_ONS_18/ON_202
The dice is just a wild guess, based on the feeling one gets from reading U-boat war diaries. It really depends on the situation and what year one is playing.