Natural carb keg

[farmskis]

I am about to naturally carb my corny. From what I understand I determine the amount of sugar to add based on the entire volume of the corny since the headspace pressure will equal the pressure in the liquid no matter the liquid volume. When I work the math to find the amount of sugar I get an amount that is close to most calculators. (3.5 Oz, if going with 5 gal corny and temp at 64 degrees 2.1 volumes.) but when I run it in Beersmith it says 1.25 Oz. Now my batch size will be more like 3.5 gal so is Beersmith assuming I am kegging in a keg the same size as my batch? Or am I missing something? I have also heard you take a percentage of an amount for bottling.