Pro-Mate 22-2-4 Fertilizer with Trimec Post-Emegent - 50 lb. bag requires that you use 3.2-5 lbs. per 1000 square feet.
If your spreader settings go from 1 to 10 (1 letting out the least and 10 letting out the most) you typically want to be around a 3 setting. If your spreader goes from 1 to 20, you should probably be around a 6 or 7 setting. The goal is to just run out of product when you are done walking the 1000 square feet. One person may walk faster than someone else, so they may place It on a higher setting than someone that walks slower.
Almost all fertilizers, herbicides, insecticides, etc have treatments in amounts of so much product per 1000 square feet. So if you can figure out what your walking speed is then you know anytime you apply any product that calls for so much of it per 1000 square feet, you always set your spreader on this setting. You can calibrate this by marking off a 1000 square foot (10 x 100) area and placing 4 ounces of any type of product of similar granule size, or of the extinguish in the spreader. Start with a low setting and start walking your marked off area at a normal pace. You can adjust as you go and find the basic setting for you to apply the product over a 1000 square feet.