In my experience it seems to be related to sea state, this is by no means set in stone but when evading escorts in rough seas PD often works better than 30 meters or so, the problem is the conning tower breaching the surface, you don't want that, as a rule I set the depth to 17 meters or so in such situations.
It's definitely not a foolproof method, sometimes it works amazingly well, other times you're dead that much sooner
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