I really dont know the 'real' answer -- I think theres a formula in Machinerys Handbook.
Strictly 'for what its worth' - your mileage may differ - I never slept at a Holiday Inn, etc, etc, this was my experience...
I had done several in 3x2" & 6x2" carefully trimming away with cross feed and carriage from a central point, trying to keep a 'rounded' profile - mounting it on the sander only to watch the belt wander (which happens when there is too much crown) or, the belt not track (uneven crown) - even scrapping a couple wheels when I got to much crown. Having heard several comments about how little crown is actually needed to get tracking and needing a 2x1" wheel, I just grabbed a coarse file, and drawing a magic marker line for center, started filing to the edges, eyeballing it. About the second time I tried the belt it tracked nicely. I have since done another 6" and a 3" using just a file and eyeballing - it really doesnt need much.