OK. bhg481's method over-estimates the mathematically calculated flow rate by just over 3%. Incidentally, when I said earlier that "the rate of increase of the volume of the cone is not linearly dependent on the rate of change of the radius of the cone base" that was incorrect. The rate of increase of the volume of the cone IS linearly related to the rate of change of the radius of the cone base: for a given size of cone, doubling the rate of increase of the volume would double the rate of change of the radius. What I should have said is that, for a given rate of change of volume, the rate of change of the radius of the cone base is not linear with respect to the radius of the cone base: as the base radius gets larger, the rate of change of radius will reduce, hence the difference in bhg481's method.