Most of the US orders I have found during my research used 500nm as an arbitrary figure. I draw circles around the bases marked on the NavMap at that size, and that does cover quite a bit of territory... I can find several references early to where the commanders were told to submerge by day, only surface at night when fully dark with no moon, etc., but I have not found any reference to the rescinding of same. Only in the patrol report endorsements do I see a reference, with statements like "... submarine spent entirely too much time submerged. More efficient patrolling, covering a larger area, can be accomplished on the surface..." things like that - and early in the war no less...
