What do you want to do with it?
If you are trying for accurate power curves it ought to be at hub height directly upwind by 2 to 3 rotor diameters. That is difficult to manage as the wind keeps changing direction.
If it is for a general idea of performance then off a bracket to keep it away from the tower and a few feet below the blade tips will probably be good enough. Make sure the tower shades the least useful wind.
Try Hugh Piggot, Paul Gipe and Michael Klemen's sites if you want to be serious about it.
Flux