Bear in mind that actual windspeed at any locality is very dependant on local terrain, especially trees and buildings. The higher the mounting tower, the less these obstructions will affect the wind. Even in completely flat terrain, windpower increases approximately linearly with height.
The rule of thumb is that turbulence from an obstruction reaches up to twice the height of the obstruction. But if your site has obstructions, remember that wind energy is ‘free’, so its Ok to ‘waste’ some of it by not having a high enough tower.
This map shows very approximate areas of wind power density class, where class 11000 W/m2.