Agree - some tracks will have a marked out 1,2,3,4 5 ... Km marks on the pavement that you can test. You will get some minor variation depending on how you ride the corners etc. I have found that I need to vary the computer settings a liitle from the roillout method by trial and error, so things like rider weight tyre pressure may influence the true reading.
If you need accuracy more than the 99.5% + accuracy you will get by just plugging in the number from your manual, then the previous posters have some good ideas.
But, in my reading of your message, it sounds like the extreme accuracy isn't an issue, it is that you don't know how to set it...
If you don't have your manual, you can find the number you need here:
For specific directions, I don't see yours listed, but you can try the methods used for some other Cateye models to see if one of them works. Those are here: