Thanks,
I think the "out of direct sunlight" may be key for my problems. I've probably varied how I've done it, and I know it's been different on cloudy days.
I'll be carefull about where I do it and see if I can get more consistant readings.
I've been trying to get my CYA level up to 50 because I can't keep my FC level up without adding bleach every night. I think that a higher CYA level protects the FC. Is this true?