I considered getting a graduated cylinder or some other container with markings but to be quick and efficient there needs to be as little thinking at the pump as can be. it's the partial or uneven volumes that kill me on the road.
Example:
I can fit 1.75 gal, with a 2% cup I have to add:
measure to 1 gal, pour it in (this gives me 2% for 1 gal)
measure to .5 gal, pour it in (now I have 3% for 1 gal)
measure to .5 gal, pour it in (if I don't get lost this could have been combined with the last measure)
measure to .25 gal, pour it in (now I have 3% for 1.5 gal)
measure some random amount for the last .25 gal and get back on the road.
when I have two tanks to fill I get to do that twice.
When I use to just blip around town it was not so bad, as you figure out how much you can fit in when you go on reserve and don't fill till then. On longer trips where the gas is not at known distances it's harder.
Maybe it's just me that can't do simple math in my head, so instead I'll do it all ahead of time.