It's like when one of my kids is going on a 250 mile trip. I remind them they need to fill the gas tank. Now, I know that all our cars can go quite a bit further than 250 miles on a tank of gas. So am I disseminating misinformation? Should I instead say, "Put enough gas in the tank so that it contains exactly 12 gallons." "Fill the tank" is good enough to cover things and give an extra safety margin. "Boil the water" is good enough to cover things and give an extra safety margin.