Can you tell me where the 15% rate is coming from as far as success rate? Maybe you are just assuming number of tables / total tables in the restaurant. I am curious though how you came up with that figure. I think your chances are better than that. Some people do not request a window seat, and some people don't even know that they can. I would suggest doing what most of the others have said:
1) Make note on your ressie when you make it that you are celebrating something and that you would like a window seat, if possible
2) Check in for your reservation early. This will allow you some extra time to wait for the seat.
3) While checking in early, be sure to request that window seat, let them know you are willing to wait for it, and repeat the fact that you are celebrating something
We have never had a problem getting a window seat when we have actually wanted one, and at least once or twice we got one without even asking. Do note that "window seat" and fireworks window seat are two different things, as part of the restaurant's windows are on the side opposite of the Magic Kingdom. However, those are great seats as well because you can see other parts of Walt Disney World from there, and of course, you can always go outside to watch the fireworks instead of viewing them from your seat. Best of luck!