I drilled my hole directly in the middle like Dave Weldon that already posted a picture. Works very well.
You could do that very easily because of the shelving you're using. I've actually seen people with that kind of shelving have the cage standing directly on the shelf with just a drip pan on the shelf below.
Edit--Dave said he drilled in the corner. Sorry, I meant to write that I drilled into the bottom of the pan like Dave did. As long as the hole is in the bottom, and the water has begun draining through, the cohesion will allow for the entire pool of water to drain. Essentially, it doesn't matter where the hole is. Even if the water doesn't begin to drain until the entire pan has some water in it, it's okay because as soon as it starts draining, it will be able drain all, if not most of the water (assuming the pan is level)