1. A method of determining a mass flow rate of product being applied with an agricultural implement, comprising the steps of: providing an air flow from a pressure source at a known air flow rate in an air line; determining a pressure drop in the air line along a known distance in a downstream direction using at least one pressure sensor, each said pressure sensor being positioned downstream from the pressure source; repeating the providing and determining steps at a plurality of known air flow rates; metering a product at a desired application rate into the air line at a selected one of the air flow rates; establishing a pressure drop across the known distance at the selected air flow rate, while the product is being metered at the desired application rate; calculating a specific pressure drop by dividing the established pressure drop by the determined pressure drop, at the selected air flow rate; ascertaining the values of parameters A and B using the mathematical expression: 
 α=1+ Aμ+B √{square root over (μ)} where: α=specific pressure drop; μ=mass loading ratio; and A and B=parameters based on measured data for the specific product being applied; and estimating a mass flow rate of the product being applied.