After just cutting my slots today, I'd definitely echo javeryh's suggestion to test on scrap. I ended up going through about 3 sheets of scrap before I got the setting right.
One other helpful suggestion is to make sure that you dont cut the slot while you are walking (i.e., moving your feet to catch up to the router), and that you are always ensuring that the bottom of the router is flush on the MDF. I moved to catch up and the bit traveled up and nearly came through the top of my CP. Good thing that I cut two CPs since that one is now scrap.