The $100/day is a general recommendation but one that fits most people. It prepares people for the expenses they may incur.
Some people will spend far less than that and some will spend far more. But yes; If you take $1200 on a 12 day cruise you should be in good shape.
The other thing to remember is that your ship's onboard charges will be charged to your credit card. If you don't plan to spend much on shore or gamble then your per day, cash expenses, will be less.
I am always amazed at how much I spend, in cash, with taxis, tips, restaurants, etc. before I ever set foot on the ship.