The spreaders are not calibrated linearly. Be sure to follow those directions. I have a lot of thatch built up in my lawn. Thatch is a layer of undecomposed stems, leaves, and roots between the grass blades and the soil surface. Dethatching or core aeration is typically performed every year or two, and the optimum timing depends on the type of grass.
Aeration is the most useful treatment for moderate thatch.
Breaking through the soil with an aeration device is easier and less drastic than dethatching, and it causes less stress to the grass and less debris to be cleaned up. A power aerator can usually be rented locally, or you can pay a lawn care or landscaping service to do it for you. Verticutting is the preferred treatment for St. Augustinegrass lawns in the South. Bluegrass lawns can be dethatched in spring or fall, though fall is preferred. In the spring, just wait until the soil dries out enough to be workable.