[captlar]

2 pennants is standard in my area of New England. Mooring balls come with one, but the vessel is expected to provide the 2nd. A few years ago we had a few boats lose their mooring (chafe - neglect) and sweep through the field. People were pissed. The marina now supplies 2 pennants on each ball and passes the extra cost along in the mooring fees. Pennants are equal in length. This has more to do with negligent owners than seamanship. With equal length pennants, the load does transfer from side to side which actually helps reduce swing. Yes - they do get wrapped but chafe guards protect the critical spots and I assume someone is checking the boat weekly so I wouldn't worry.

The safety pennant of longer length sounds more like a storm pennant where it is a temporary back-up. Marina will put them on when a blow is coming.
My everyday pennant is 2/18' of 3/4".

Some fields are set up with single chain moorings, which do provide some extra scope, but other fields use a 3 point mooring and the only slack is to accommodate tide. Best to ask what the set up is.

Jon - I don't know any formula other than more is typically better. In my marina, they have us packed in (if the guy on the mooring has his dink tied off the stern, you could not fit between him and boat behind) and specific moorings will only accommodate specific length of boat. I would look at adding a new chock or fairlead further forward specifically for mooring to try and keep pennant up and away from striker.