If your blue waterline isn't visible anymore, it's about an inch from the top of the unit. On my 120 it makes about 1 to 1.5" wave but a 125 is longer than a 120 so you can probably make a bigger wave than I can. I've seen them placed in back corners, front corners, and sometimes on the back facing forward, like chingchai's from thailands tank.
I personally have mine in the front left corner and my corner overflow is in the back right corner. It's best to have your rock off the glass so the water has a race track. Here's a picture from a couple weeks ago, you can see the water line on the front of the glass to see the size of the wave it makes on my tank.