You calibrate the spreader. Then you begin to fertilize. Every time you use the spreader you keep track of how much fertilizer it took to do the size job . If it gets off by more than a small amount you recalibrate the spreader. Use the same fertilizer after you calibrate. The settings mean very little on any spreader as they vary to much. Most esp. cheap spreaders like Scotts. There are many articles on spreader calibration. KState has a couple and so do most extension programs in most states. I never go by the settings. I just measure the amount needed for the size of ground dump it in the spreader box and set the spreader very low. Then I go over the area several times until all the fertilizer is gone.