Immersaview has a screen design utility. But I've not used it in awhile. I'll ask Andy and Alex later today when they get into the office. I'm curious of the status of the application myself. I mostly just use Projector Central's Projector Calculator and drop in the information of the projector I'm researching specs on to get a rough idea of distance requirements to screen.
It works for the shortest distance to the screen (which is the edge of the projection not the center). The center of the projection is further away than the distance for a flat screen. The information for Projector Calculator is useful, it's just that you have to use it slightly differently for a curved surface. It's easier to explain with an illustration. I'll look around tonight and try to find an example of what I mean.