Slow shutter speed you say, i kind of already know that much, there is a huge variance from one end of the scale to the next...1/2000 - 20 secs. I am guessing start at 2 secs or so and work up to 10 in 1 second segments. would 2 secs be a good starting point? I am presuming the longer the exposure the softer the water will become to a point where it is no longer effective..just a wash of white! Matrixx