I needed something quick for how much water the model person would use. That was "an average person needs eight cups of water a day".
8 cups / 24 hours = 0.3333 cups/hour
A quarter of that is 0.0833 cups / 15 minutes.
1.000 litre = 4.227 cups
0.0833 / 4.227 = 0.0197 litres
But I'd needed half a litre, not a litre in the conversion, so the number is doubled for 0.0394.
As a percentage, that's rounded to 4%.
@shadowfals @chaosmonkey
Nicely done. I hope @chaosmonkey is satisfied.