Originally Posted by chadbraun
I would go with the 2.5 oz if I was you. With a irish stout you want your volumes of CO2 to be around 1.6 - 2. With 2.5 oz of dextrose you should get about 1.8 volumes of CO2 in a 5 gal batch. If you add the whole 5 oz of dextrose you will get around 2.8 volumes which is out of the style guidelines.
I agree, mostly. I'd still want the stout to be on the low end off carbonation, so using the whole packet does seem excessive. However I would shoot for 2.2 volumes of CO2. 1.8 volumes is pretty low and I believe most bottled stouts are higher than this. I also think anything under 2 volumes is tough to appreciate if you are not used to it - and most folks have never had a beer this low, so it is better to err on the higher side of low in this case. Just my $0.02 anyway.