rulers are nice, but when you want to tell the difference between 470mm and 469mm, there is no better way than with a stick that is 469mm long.
The reprap build instructions suggest you cut some sticks of precise lengths in order to verify that your threaded rod lengths are exact when the frame is all bolted together. That is a good idea, but it seemed like a little too much effort. What I did was to use the last threaded rod from the reprap build. Before I installed it, I put two nuts on with a washer in between. Now I can move the washer to whatever length I need to measure and I can stick it between two vertexes. It’s less steps, and it already uses parts that you have!