Sounds like you could use something on the underside of the ruler to keep it from slipping. There are a few different things you can do/get. I have clear non-slip dots on mine. They sell a roll of stuff, i forget the name, that you can put underneath. I think it's like a cling or contact paper. I'm sure someone here can come up with the name. I've also heard you can put a few dots of clear nail polish, but haven't tried that one myself. Small pieces of adhesive backed sandpaper can help. I didn't like that though, mine peeled off.