Thank you for dropping by! I have a hitech nd-grad set with the LEE filter holder(1,2 and 3 stop) and a kenko nd 400(9 stop). Depending on what i want to create and the amount of light available i will use them differently. For extremely long exposure i will stack the kenko with one of the hitech (windstorm or frozen for reference), or use the kenko alone (the reflection).
For general seascape, because i usually shoot during sunset, i will pull down the nd grad so it becomes a full nd to have an exposure long enough for streaks to form, followed with another nd grad to balance exposure. Number of stops used will depend on how bright it is (any seascape pic for reference). Reflection just depend on how still the water is.(any reservoir shots). Otherwise you can create them in post production, although i find that they look fake.
To be honest, depending on what you want to create, you may not need to use filters. I started without and just took multiple exposures and blended them manually in post production(which i still do sometimes).
Not that i need it, but just want to 
