Usually the markings are laser made for accuracy.
I asked my plastics company guys and they don't do markings for that reason.
One friend is getting some acryllic and using a metal straight edge then will use a nail point to score it on the diagonals for the markings. It will be a square up ruler for her hand embroidered squares.
When I was looking to make some specialty rulers I searched for a company to do it and found one. Unfortunately, the set up cost is quite a lot and I didn't have it at the time.
Anyone know of how to do this without damaging your budget please let me know.
ali