Testing Liquid shock

I have a question about some liquid shock I just bought, 4 gallons of 12.5 liquid shock for $2.50. I added 1 gallon to my 8000 gal pool to bring it to about 16 PPM which is just about right for my CYA of 40 - 50. I only semed to raise my CL to about 3 PPM. I figure I've either got something really nasty in my pool or the shock is really old. Does anyone now how I could test the shock to see how much chlorine is really in it? I was thinking about something along the line of diluting it with distilled water to a certain point and then testing that but I don't know how much to dilute it and what it should read. Anyone know how to check this?

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.

Thanks, I'll try this right now.