Okay, so my recommended FC is 3 to 7. I checked it this morning using the OTO test block. Color is a perfect match for 5. So I checked it using 25ml FAS-DPD. 5.0 FC, .2 CC. Perfect.

But then I got to wondering....why don't they make a color block with useful FC levels? All I know is if I'm close to 5. If I'm way over, I can usually tell I'm above 5. But is it 7? 8? 10? Who cares if the FC is 1? That's not really useful. Who runs the pool that low? Why not have color blocks at say, 2, 5, 8, 11, 14?

Then I had the thought....if I only put in 3 drops instead of 5, my 5 reading should show up as 3. If I read 5, that should indicate rough 8.3. So I tried it. With three drops it still reads a solid 5.

Sigh.... I don't have the heart to try it with 1 drop, or 2....

Why doesn't someone make a color block with useful FC levels?

Number of drops has nothing to do with it, as you discovered. The dye turns the same color no matter how much you add.

There are two things going on, one is just historical accident and laziness. The OTO test has never had other colors, so no one ever adds any other colors. The other is that it is extremely difficult to tell the colors apart between 3 and about 10. Most people can't see the difference, even with a good color reference.

Ben Powell used to talk about having a color standard made so you could measure shock level with an OTO test, various shades of orange and red/brown. The results would be +-4 or there about, but it would be a simple inexpensive way to measure shock level. Unfortunately, making color comparison charts is quite difficult and no one has gotten around to doing it.