You know, this test has me pretty confused, I've gotten quite good at all the others except for this one. There is a point where the black dot almost dissapears, but you can still barely see the outline of it. From there to where it is completely gone is quite a spread, like 20-30ppm. So I'm not sure whether I am at 60-70ppm or more like 30-40ppm, and it makes quite a difference in how much FC I should maintain. How should I judge this test?