I use a Hoya NDx400 9 stop — “Big Stopper” — ND filter. It’s nearly impossible to focus once the filter is on the lens. Computing the exposure values for the use of an ND filter can be complicated. Slower Shutter is a simple app -- in appearance and functionality -- designed to assist with long exposure photography.
Using Slower Shutter is easy. With your camera in Aperture Priority mode set your desired aperture and ISO. Your camera will determine the shutter speed. Take note of the shutter speed. Enter the shutter speed value into Slower Shutter. Select the strength of your Neutral Density filter into Slower Shutter. Slower Shutter will display the new shutter speed that you'll need to set the proper exposure for your photo. If the shutter speed is greater than 30 seconds you'll want to set your camera to "bulb" mode and use a remote trigger. Use the timer on Slower Shutter to time your exposure.
Long exposure photography isn't difficult but if you need help the, Slow Shutter's developer, Mike Wong, has written a simple tutorial.
Slower Shutter is $0.99 in the App Store.