It is dependent on your focal length. I've captured spey casts with shutter speeds as low as 1/400th of a second with a 24mm focal length. I don't know of a formula, but multiply your focal length by twenty to get your min shutter speed seems reasonable enough. eg 24mm X 20 = 1/480th sec minimum. As far as focal length, a 15-35mm range should be fine for most spray poles and lines. However keep in mind that sometimes a little motion blur will help an image pop more than just outright freezing the motion.