Henry's Law and Volumes CO2

I'm trying to use Henry's Law to determine the Volumes of CO2 in a fully fermented beer by volume of beer and temperature in the fermenter.

I was using the chart mentioned in this article http://www.brewery.org/library/YPrimerMH.html but when I do the calculations I get different numbers than the chart shows. My only guess is that the value I'm using for pressure or partial pressure is incorrect. What should I be using?

Also, other calculators I have used give very different numbers in grams of Dextrose or Sucrose required than this article does, but the calculators don't explain what values they are using to determine the outcomes. Is the amount used in this article (4g/l Sucrose = 1V CO2) a good approximation?

Thanks!