Help with wind tunnel experiments

In our wind tunnel experiments, we have wake survey of a cylinder and boundary layer measurement for a thin flat plate as two experiments. We measure the stagnation pressure(Pt) at points using a pitot tube. We also measure the static pressure of the free streamPinf.

But while calculating the velocity at any point, we use thePinfas the static pressure at that point i.e we say Pt = Pinf +0.5(rho)v^2 . My prof said we are kind of cheating here but we can get away with this. He also said that the reason we can get away with is different for the wake survey and the boundary layer measurement experiment. But he did not tell us the reason and asked us to find out. I tried to find out but could not. Can anybody help me?