I cannot help you for that problem. Settings are sort of arbitrary--they don't have much linear relationship to the amount applied; the two spreaders types operate in different ways to release the product.
The proper way to calibrate the amount applied is to weigh the product--apply to measured sqft area--adjust up or down; repeat until an acceptable setting is discovered. Different products or seed types will require a new calibration.
The coverage should not be any different. The number of particles per square foot should be the same. A rotary spreader will have a tapered edge pattern which requires about 20 percent overlap on each side. A drop spreader will have a discrete sharp edge. Both are sensitive to the speed of walk.
I suggest, add about 20 pounds to the spreader and weigh it (by standing on your scales if necessary). Apply fert to accurately measured 4000 sqft. Weigh again; find difference. Calculate the actual rate, adjust up or down, and try again. Use a stop watch to keep the speed constant. Be sure to keep the swath width constant (measure it). If you applied to a cement floor--sweep it up for reuse.
Some companies make calibration catch baskets. You simply weigh the amount present in the basket--no need to sweep it up.