Guidelines such as the City of London Microclimate guidelines suggest 400m from the center of the site unless there are any particular large buildings outside that distance that could play a significant role on the flow.
Based on our experience, for early-stage design, it’s acceptable to include context less than 400 m from the center. However, in a case where the site layout does not conform to a circle, i.e. if a side is 800m long, only 100m wide, there will not be ~ 400m of context around the top and bottom of the site which will impact the results.