There are different psi per each style... one of the more common styles wants about 2atm's which I think is about 11psi... but you should look for a calculator of some kind... I'll edit when I find one I know of...
Edit:OK here's a good one:
http://www.tastybrew.com/calculators/carbonation.html
I also use my built in one that came w/ BeerAlchemy for my mac, I think other apps for brewing would contain that too, but not sure...
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