I agree that it would be helpful to know how much beer is recommended. This page (https://www.diffordsguide.com/g/1150/how-to-make-cocktails/cocktail-glassware) says a Collins Glass can be 34cl to 41cl, so, depending on which size glass you're using, we're talking about a variance of 6cl if you "Top up with" the beer, which is quite a substantial variance...