Here is what I do. I have a good idea of the setting range I need for my spreader for the common SGN ferts out there. For my Lesco 80 lb, it is between 11-14. Then I use my OWN lawn to calibrate since I have a measured square footage of my lawn and don't care if I come out a little heavy or light on my own lawn. Then I tweek open or closed based upon my results and bam you are ready to go.
Not scientific, BUT you don't waste all that fert spreading over pans and making a huge mess and accuracy is still pretty good.
7x14 Stealth SE Enclosed with 48" side ramp
2013 Gravely Pro Turn 260 with Kohler 26.5 EFI
08 Toro Z400 48" deck, 21HP Kaw with EZ Vac twin bag
09 Exmark Turf Tracer HP 36"
21" Toro SR4 Super Recycler
Stihl KM90 Kombi w/ straight shaft trimmer & tiller heads
2 Husqvarna 125B Blowers
Turfco XT5 Aerator
Turfco LS-22 Overseeder
EcoLawn 200 Applicator
I can do all things through Him who gives me strength.