Reading Salifert Ca test..

[Son Of Skyline]

When I test Ca using my Salifert kit, instructions say to keep adding drops until the stuff turns from purple to clear blue. Problem is, it doesn't just flash from purple to blue in one drop, but it slowly turns a little more blue with every drop, and when it does seem to turn blue, it goes back to purple within a minute. At what point am I supposed to stop the test?

Thanks.