Can you take it to your fertilizer dealer and put some in it then run it out without spinners  into loader bucket and weigh loader? Dry fertilizer is a tough product to be consistent with. The spreader is designed to meter volume and every product has a different density. Even the same product has quite a bit of variability from one load to the next. If you can consistently get within 2% accuracy you are doing good. Scales on the spreader is the best way.