If you usually fill up at about the same level every time, measure enough for that. As long as you're fairly close, I doubt it matters too much. Like DmaxMaverick said, you may be wasting a little, but probably not enough to break you. Maybe you use a little less next time.
I just tell myself it averages out in the long run.
If I was only getting 3 gallons of fuel, I wouldn't use a regular "dose", but 1/2 tank +/- a couple gallons I'm not gonna worry about it.
Dave
2005 Silverado 2500HD CC LS Duramax/Allison