Here is an outline that I wrote up years ago. Note that this file is in .PDF format. To view or print it, you need Adobe Acrobat Reader installed on your computer (it probably is). If you don't have it, you can easily download Adobe Acrobat Reader.
To calculate the amount of fertilizer you need to put on your lawn, you need to know the square footage of your lawn. It is simple if your lawn is rectangular but can be more difficult for circles and other shapes. After years of calling up my wife to ask her for help on calculating the area of my weird shaped lawns, she wrote me a program. Simply put in the measurements asked for and it will calculate the area. Besides calculating area, it will tell you how much fertilizer you need to use.