The distance between opposite corners should be roughly 31 feet 8 inches.
The center will be 17 feet 10 inches from all corners.
Place you center plant at that point, and the measure out from center, toward the corners, 7 feet 1 inch.
Now you have five locations, forming an "X" centered in your space.
Think of the four outer locations as a square.
Measure five feet from each corner point in line with another corner, to find the other four locations.
This places the nine plants in a five foot grid, with one plant in the middle, and the other eight around the perimeter.