Haida released today their mobile app to setup easily your long exposure. Just select the filter, set the exposure without filter (given by the camera) and app will tell you the correct exposure value.
“Haida App is not only ND exposure calculator for photographers when they take photo outdoors, you can also review famous photographers’works and photography’tips and experience sharing .
calculate your exposure time when using Haida ND filters. Select your base/normal shutter speed before adding a filter. Select your filter and your new exposure time will be calculated”
Google store download link