Well, I'm basing the 20ppm on the chart here: http://www.poolforum.com/pf2/showthread.php?t=4236 At 30-50ppm, it looks like I'd need FC of 4-6ppm. I guess that's not a huge difference, but I'd like to keep my chlorine level as low as possible.

As for the test, are there any specific guidelines on the conditions under which to perform it? Do they assume you're testing outside? Should the beaker be in direct sunlight? I get a big difference in results depending on whether the sun is overhead or is low in the sky. (Light shining through the side of the beaker makes the dot more visible.)

I don't know... that test just seems goofy to me. I can't believe someone hasn't come up with a better way. The CYA strip test changes color. You'd think that someone could make a dropwise color test for CYA.