From the book I mentioned "Practical Arduino" :
"There is a slight complication though: most flow-rate sensors do not have a consistent scaling factor across their entire operational range.
At low flow rates the sensor might be impeded more by friction in the bearings, so its output frequency could actually be lower per liter than at higher flow rates. That variation could be corrected in software by applying a different scaling factor depending on the measured pulse rate. However, because the accuracy of inexpensive flow sensors is typically only +/– 10% anyway it doesn't really matter much in practice that the scaling factor deviates slightly at low flow rates."
PS: you can also find his sketch on the link I posted