To use the 40 point method for solving for the antenna height above average terrain (HAAT) you need a good topographic map with 10 foot or better contour lines for a distance of 10 miles from the antenna site. Check the links on this site for a map that you can print.
First determine the antenna location and elevation on the topo map. Enter the elevation and antenna height in the calculator.
Draw concentric circles on a copy of the map at 2, 4, 6, 8 and 10 miles from the antenna. With a protractor or other method draw bearing lines from the antenna at angles of 0, 45, 90, 135, 180, 225, 270 and 315 degrees.
Where the bearing lines intersect the circles read the elevation from the map and enter into the appropriate box on the calculator. Use only whole numbers.
When all values are entered, press the Solve button to determine the HAAT. Good Luck!