Yeah I agree experience could (should?) be employed here
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I just asked because the other day I sank a large cargo and a large merch in a convoy. At the the time of targeting I'd expected them to be about the same but the cargo tunred out to be 9500 but the merch was over 13000, a much juicier target!
Since then I found another one of these large merchs sailing on its own. Enjoyed stopping it with one torpedo and then sinking it with 5 shells from the deck gun. Nice rate of return - if only every torpedo got me 13000 tons!