66 is a multiple of 11. So to make your decreases come out even you would k9, k2tog around and then decrease in the same manner as you stated.
To figure this out - If you add the 9 + 2 you get 11 and 11 is a multiple of 66. After I figured out what multiple it was I subtracted the 2 for k2tog and I ended up with 9.
Some numbers (62 for instance) don't work out. Always try to CO a number that works for ease of figuring it out. If you did happen to CO 62 you could throw in a couple decreases or increases to bring it to a number that you can decrease evenly with.
Make sense or did I totally confuse you? It took me a while to figure this out so keep practicing and don't sweat it. It's just a hat.