Mastamoon,
Here is a pretty handy link to an online pool calculator that I found in a thread on this forum somewhere (I think). It's helpful in getting an idea of how much to add. Most importantly, don't just throw the whole amount in...its not 100% accurate but it gets you damn close. Add some (half?) and then retest before adding more, if needed. Just like Watermom said above.

http://poolcalculator.com/

I hope this helps!

JP