u can go with the 2/3cup for 5g batch, but different styles of beers call for different carbonation levels. Here's a list of general CO2 levels for beer styles. BTW, 2/3 cup is about 3
Style & Volumes of CO2
American ales 2.2–3.0
British ales 1.5–2.2
German weizens 2.8–5.1
Belgian ales 2.0–4.5
European Lagers 2.4–2.6
American Lagers 2.5–2.8