Tough question Jim!
There are a whole host of variables involved here that I doubt you'd want or need to go into, but they include the diameter and width of the pulley, the type of belting material and its width and thickness, plus the span between the pulleys, plus the crown or lack there of for the other pulley, and no doubt speed and power factor in as well.
There is also the form of the crown, whether it is a smooth curve, a simple ridge with flat lead in angles, or a flat across the top with just the edges forming tapered angles.
My feeling is that if you crown it just enough so that you can actually see the crown, that will likely be about right .... say about .015-.020 per inch, and if that doesn't do it, add a bit more.