I hope I can make what I am about to say so it is understandable.
When I am after accuracy, I will use 2 rulers.
Line up the one that I am going to use my rotary cutter against and the second one backed up to the first one to make sure it is where I want it to be. A big square ruler works good for the second ruler.
This works especially good when cutting widths of fabric that you later going to sub-cut.
Joyce