How does one determine the freestream and ultimate slipstream velocities to estimate thrust?

The task before me is estimating the thrust of a propulsion system (a ducted fan) using this equation for thrust (Eq. 2.1.1)1:

$$T = (mass\; flow\; rate)\;(slipstream\; velocity - freestream\; velocity)$$

I know what the ideal inlet and exit velocities of the system are, but how does one determine the freestream and slipsteam velocities so that estimated thrust may be calculated?

1 Warren F. Phillips, Mechanics Of Flight Second Edition, John Wiley & Sons, Inc., Hoboken, New Jersey, 2010, p.153