I used +10 on my 16s, I took an old extension cord and routed it with existing clutch allowing for the width and height of the new bars and measured the cord to get the right length. I did the same for the break line.
The easiest way to get the cable length right is to install your bars and then measure for cable length. I did my bar swap during winter down time so I wasn't worried about the extra time it took to do this.
Absolutely the wisest choice of all...no guesswork...because if ya guess wrong, you'll use all that time you saved and more by procuring the right parts...
And, it may amaze you guys who aren't going to extreme heights on your bars just how much slack there is in the stock cables and brake line! For your brakes, it wouldn't be a bad idea to convert to an AN-3 stainless system...thataway, if you change bar sizes in the future, you won't have to purchase an entire brake line set...just replace the line between the master cylinder fitting and the spider...
Correct me if I'm wrong please, but doesn't the newer bikes 08 and up have the brake lines that disconnect at a junction box? At least that's what it seems from what I'm seeing in the brake cables for 07 and older. Figured I'd just buy an extended line for 08 or up, and make it work.