A tip to help with accuracy using rotary cutters:
I have my cutting table against wall and square up my ruler with the wall. Then I adjust the fabric beneath the ruler. When I cut I push the ruler against the wall and it doesn't move. I have found that otherwise it seems to move just when I am almost to the end of the cut, making my cut off just a bit. I need all the help I can get!!! and this works for me. LOL.