You might want to double check the time on your docs. If I remember correctly, I boarded from the lattitude line last year about 11:45 and was on the ship minutes later. The boarding process was quick & efficient.
I don't remember there being a set time. Ordinarily on domestic cruises general boarding is about 1:00 (see your docs) but they will allow boarding as soon as the ship is cleared. I've gotten on as early as 11:30 and usually an hour or so prior to the printed time. Latitutes will have a special check in counter which will help speed things up on the front end but I don't recall if they actually announce boarding for Latitude members. Seemed as though we just a spot closer to the front of the cattle shute. On some ships I seem to recall they boarded by cabin numbers/decks but I think it was irregardless of Latitude status.
I know you're excited, but just take a few deep breaths, close your eyes and picture that little umbrella drink at the top of the gangway!