I was taught to use my ruler for measuring and cutting, not the mat. Not to cut on the lines of my mat, so as to not wear it out. I do feel for you. I once measured for blinds, had them made by Three Day Blinds. They were too short and too narrow. I went back to them and they remeasured - they were exactly what I had ordered. Went home and checked the measuring tape I'd used against all the others in the house. It was a bad measuring tape!