I'm a bit late to your calibration question, but let me add what I do. I work at a research institution, so I am a bit more anal about this.
Spreader calibration has a modest learning curve, but put a bit of time in up front and you will be set to do accurate applications with only modest input to recheck or do new materials. Some spreaders are crazy off on spread pattern and you read about guys here applying in clockwise circles to compensate.
I do the pan method on a large tarp to adjust the pattern AND to determine EFFECTIVE SWATH WIDTH. The problem with the CUB and Lesco vs. the Anderson is that everytime you change the (3rd) hole to even the pattern, you also change the flow rate (amount). The Anderson's cone system doesn't.
Ideally, you want even left/right patterns. I have nifty vials to compare the pan collections like this:
Learn to walk consistent speeds and hold the spreader near level (within reason)....this can change calibration.
Once I have the swath width and a uniform pattern, I do the calibration amounts like this:
Prizelawn has a calibration box to collect the material.
I.E., 10 ft. effective swath.....run the spreader with box 100 ft. = 1000 sq. ft.. Adjust and remeasure until satisfied. Keep a general track of your spread per bag to make sure you are in the ballpark.
I DIDN'T BUY THE $250 PRIZELAWN BOX. I MADE ONE OUT OF CARDBOARD AND DUCT TAPE (Red Green style). Check out the Prizelawn manual with calibration instructions here: http://www.prizelawnspreaders.com/vi...?itemId=EST-17
In effect, this lets you calibrate a rotary unit like you do a drop spreader.
Once made, this method is quick, simple, fast, and very accurate (within reason).
O.K., flame suit on, everyone go ahead and take a stab.