Salt or Fresh

[helopilot2be]

I have just purchased a Puck Air. A what point do I switch the setting from fresh to salt water. I understand in the Ocean set it to salt. In the pool or a lake set it to fresh. I plan on diving in the Chesapeake Bay. The Southern Bay is very salty the Northern Bay not so much. Is there a general rule or does it make that much difference. Thanks for any guidance.
Chris