I guess I am old school, but I alway size for the waist of the ski. I hate when I fold them up and I get glue sticking everywhere. That said, I would probably aim for 100 wide to minimize that issue. I agree with Matthew that the tip does not get you much traction anyway.

When sizing up a skin for a ski you want as much coverage as possible. With that being said, we're working in millimeters, a very small measurement, and most of the traction you get when skinning up comes from the last 3/4 of the ski. The tip really doesn't help generate any traction. A 115 skin will obviously get you great coverage underfoot, and in the tail, you'll have 2 mm on either side of the skin, which is basically the thickness of the edge, and material you would have trimmed away anyway if you were cutting the skin to size. And in the front, you will end up about 5 mm on either side, hardly a large amount. It appears that by getting the 115, you end up with an almost perfectly fitted skin without having to do any trimming except in the middle part of the ski to fit the waist. If you went for the next size up, you would be spending more money and cutting away a lot of material. The 115 is the perfect size. Go for it. Here is g3's guide for choosing a skin. You will see the 115 fits in perfectly for the size ski you will be using: http://www.genuineguidegear.com/service/g3-learning-centre/skins