You can play around with it. If you enter volume as 2.5, gallons as 4 and temp as 65, you get 4.28oz. So he probably put in around 2.5 for the volume.
So if you want a mild carb level then 2.75oz should work fine.
I used that calculator when I did my hefe, I wanted high carb volume and ended up using like 8oz corn sugar, worked great.
Drinking: All kinds of stuff
Kegged: Rollings Hills Pale Ale
Conditioning/bottled: Rollings Hills Pale Ale, Clackmananshire Wee Heavy
On Deck: Oldenwald Dunkelweizen.