You will have 2/3 formal nights and if you are happy with an evening dress fine but nowadays smart dress/trouser suit is more the thing. You are on the Balmoral not the QM2, so relax and wear what you feel comfortable in. There will always be some who will wear an eveing dress but that maybe there way of doing things but not always everyone elses.
About to go on QM2 on the formal nights we have some theme nights to you have to get dressed the part or can you just go in your tuxedo instead of pirates gear as they say no jeans allowed !
You will be sent the "dress code" with your tickets so you should have no worries.The norm on Fred. Olsen is two formal evening each week.Remember there are also a few "theme" nights eg tropical, country & western, British (red/white/blue)so it would be worth considering something appropriate. Have a great cruise on a lovely ship.