Roland, FYI U.S. Army Field Manual "Tactical Employment of Herbicides" FM- 3-3. It talks about three (3) types of tactical herbicides; Orange, White and Blue. It also mentions the "drift area" of these herbicides and states "a 500 meter buffer should be maintained to protect desired vegetation". That more than covers your 20 or 30 feet, as it is over 1640 feet! So, add that to the perimeter. LouB.