I used the RS Sensor also. I taped off about 1/4 on each side. I'm new to this, but my first set of pics came out pretty nice. I covered them up with folded over duct tape first and walked tested with the test light until the sensor would trip Then I put a garbage can there...did the same on the other side. I have my camera mounted in the box so I can look through the view finder. I was able to see both trash cans and figured that was about right.
Actually I have all the Canon sensors exposed behind the glass. Says here you don;t need to, but thought they are all on the camera for some reason...might as well used them.