This is what you need to do if you want to use your DSLR and AutoND in manual mode:
Filters can be stacked (if your filter holder has more than one slot). For example, if you combine the filters 0.3 and 0.6, you get the same result as if you use a 0.9 filter.
When shooting a sunrise or sunset, the horizon is the brightest part of the image, the upper sky is a bit darker. Therefore you need a neutral density stripe filter. You can create one by simply combining two graduated filters.
AutoND will give you a recommendation for such a combination as well.
AutoND will base all filter recommendations on the assumption that you want a maximum of five stops of dynamic range in your picture. However, you can modify this value in the settings. So if you prefer more or less stops of dynamic range in your picture, just change the setting.
This can also be useful if you think that the recommended filter densities are somehow too high or too low.