Doing the calibration right
Numbers are only as good as the pass that made them. Penn State's method: use a tarp of about 100 sq ft (10×10, 9×12), weigh it empty first, and make three passes — one straight over the tarp's center, then one on each side at your normal overlap spacing, all at the speed you actually spread. Subtract the tare, run the math, then repeat the whole thing once or twice and average — spreaders are not precision instruments. Recalibrate when the manure changes (bedding, moisture, source).
For the load-count method, the weak spot is "tons per load." A spreader's rated capacity is a volume number, not your manure's weight — weigh a loaded spreader (truck scale, feed-mill scale) or at minimum weigh a few bucket loads and count buckets.
Knowing tons per acre is step one. Turning that into nutrient credits (lbs of N-P-K per acre) properly requires a manure analysis and nutrient-specific availability factors — that's a separate tool we're building carefully rather than badly. Until then: your state's nutrient management guidance or a manure test lab is the right stop.