If you just use the settings on the bag because your spreader is listed, you could still be over or under applying. Chances are it is close, but it still can be off. Bouncing around on trucks and trailers and just normal use will knock a spreader out of proper calibration.
You should fill a hopper with a weighted amount of product, apply it to 5,000 sq ft plot, then weigh the product again to determine the actual amount applied to that plot. Check it against your spreader's manufactures settings. It might just be a simple matter of your walking speed, or you might have to use different settings when applying if your setting openings cannot be adjusted, but I would think all commercial units are adjustable.
The bigger problem that most of us should check is the spread pattern of our spreaders. You can get kits from Lesco for this or you can make your own. All you need are some small 12"x12" boxes about 1-2" tall. The cardboard box trays that soda cans come in can work for this. Place them in a row and run your spreader over them at a 90 degree angle with just enough room between the middle few boxes for your spreader tires to pass through without hitting the boxes. You want the boxes out on each side as far as your spreader will throw. You might want to go over them, while operating the spreader, about 2-3 times in the same direction, not back and forth.
You can look just at how much product was collected in each box, showing if it is heavy or light on any one side. The kits come with tubes, or you can make your own, that you can pour the product, from each box, into a coresponding tube that is in relationship to where each box was on the ground. Standing the filled tubes up, you can get a cross view of what your spreader's pattern looks like. The ideal pattern is one that tapers off evenly to each side and is more equal in the center. Using this test, you can then adjust the third hole to get the pattern right.
Using the boxes, you also get the effective distance of your pattern. By using these two calibrations, you can know how far exactly you need to be from your last pass tire tracks to get the product applied to overlap the tapers of the ends of the pattern to get a even applied amount.
I know that Lesco has a gauge or key kit for checking the pattern calibration, but the keys do get worn as well as everyone could push the key further than the next guy, so checking it in this manner makes sure it is correct despite the keys.
Hope this was informative to some and not to redundant to others.
West Lafayette, Indiana