Hint: This is the old Version still available for those of you still using iOS6 or earlier versions.
If you are running at least iOS7, take a look at NDCalc2 - the all new Version with lots of great features.
Screenshots are german but it's translated to english too.
You know the time without the filter -how long do you need to exp. WITH filter? Sounds easy but tell me quickly how long to exp. with an ND 1.8 if the time without filter is 1/320?
Now you can!
Set Filter and exp. time without the filter and you can see the correct time. In the example above 1/5
Even easier it shows you 5 because this is what you can set on a dSLR. Smaller letters showing you the actual time. For 12500'' it shows, that these are 3h, 28m and 20s.
For times of 4+ seconds you get a countdown button to easily stop longer exposures manually with a remote shutter release with alarm and vibration (if supported).