On my past Carnival cruises I've been able to get away with cocktail attire for formal night without feeling to underdressed. I've heard on HAL it's much more formal, and full length formals are expected.
If your cocktail dress comes midcalf you should feel fine. This formal thing is something my husband & I like to do just because we don't really have a formal 5 star restaurant nearby. The Captain' Reception - the second night out is usually the most formal. I found one of my formal dresses on a Dillard's sale rack for $17. For informal nights I have my little black dress or black skirt with dressy top. There once was a couple who both wore black leather to the reception. We will soon be on our 4th HAL cruise to Alaska in May.
My wife says you will be fine. She notices this stuff. We have more than 50 days on HAL and the formal dress will vary quite a bit. For example, for men, probably no more than 35 to 40% now wear Tuxes, the rest wear dark suits. We are going on a long one soon, (HAL) and I'm leaving my Tux in the closet.
Landlocked in Denver, but cruisin every chance we get.
Polynesia, Carib. '86
Cr. Odyssey, Scandinavia, '91, 30 Day S Pac. 2002
Crystal Harm, Aust., N.Z., '94
Royal Odyssey, AK,'96
Old Cr. Pr. Canal, '97
RCCL, Carib, 1998
Volendam, Car, 2000
Ryndam, 35 day S. Am., Antarctica, '03
Is. Pr., Canal, 2004
Statendam, 34 day China, Japan, AK '06
Cr.Pr., Carib. 08
Eurodam, Atlantic, Med. '10