i thought of another.
Find a ruler (brand) you like. Most begin with the 6"x24"
and stay with that brand unless you need a special ruler for a project.
All rulers are not created (printed/marked) equally.
Use you ruler for measuring and not the marks on your cutting mat. I took a class and instructor said to lay the ruler so edge of the fabric is on the left of the ruler line. This assures accuracy in the measurement and it works.