Next time, fill the can, then pour off 3 fingers in a highball glass.
Easier said than done, when you are pouring from a 5 gallon can to a 1 gal, 4 oz can, I ended up using a 20 oz measuring cup I had, that was 6 measuring cups and then 8 oz.
The reason for this is that they are all made you know where, and as the world is based on the imperial system or the metric system, unless it is American made, your not going to get an American gallon size container. Used to be that we always needed to convert Canadian (imperial) gallons to American whenever we hopped across the border, now we have to convert liters to American. PITA all the way.