Yep - agree with all this. Just go slow.....you can take away, you cant put back Just pay attention to your pressure. Dont go Rambo on it and you'll be all good. Paint pen is a good idea!That will definitely do the trick. Just keep the bit moving along the length of the edge you're working. Don't let it sit in one spot. Also, if you use a paint pen to mark the amount of material to remove, it will serve as a reference to know when to stop and end up with a clean and symmetrical window.