I did a little research on this.
Here's the thread:
http://www.poolforum.com/pf2/showthread.php?t=4781
In short, 1ml of your chlorine in 10 liters of water, or 1.88ml of chlorine in 5 gallons of water will give you, in ppm, the percentage of chlorine. So regular bleach should read 5.25ppm of FC, 12.5% should read 12.5ppm.
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