You can guesstimate if you'd like
If you are doing it in your kitchen
you aren't doing it out in the sun with your back to it (the sun) as is recommended. You results won't be as accurate but it's like rounding up or down. As long as you are doing the same thing the same way each time you have a standard and the results should be repeatable. Your reading might not be as accurate compared to the actual physical CYA concentration as out in the sun but if it's repeatable and you are at a level that protects your chlorine from being burned off ... you're good.
How to test CYA :
http://www.troublefreepool.com/content/165-getting-started
It can take a week from the time the CYA is
completely dissolved for it to show up so wait till Sunday or better yet Monday until you retest.