When I started using a rotary cutter I had big problems with my ruler slipping. I was cutting nasty squares and nasty slices in my fingers.
I treated myself to a June Taylor-Shape Cut 12" at Walmart.
It has been the answer to my having accurate (blood missing) squares. I would love to have a larger one so I don't have so many folds in the fabric. If you get one, go the largest size they offer. It will pay for itself with one mis-cut on an expensive fabric.
Good luck.