In 0.2.2, I've replaced the setting entirely and you can now directly assign the capacity of the normal-quality bucket. (valid values are from 10 to 500,000)
The reason I picked the capacity values that I did is that I don't want it to be too annoying to move fluids out of vanilla Storage tank (and a lot of modded tanks are much larger), but there should still be an incentive to get quality Buckets.
So the normal Bucket carries 10,000 (you need three to empty a Storage tank), and Rare is the first quality that can hold a tank's worth (30,000 > 25,000).