it really depends on how bright the scene is.
Hard to say.
You need to know how slow a shutter speed to make the water flowy, then work backwards from there.
I think if in strong daylight (eg. mid-day), you'll need a VERY dark ND filter
for example, if it is bright bright sun, versus overcast day, naturally the latter will require a less strong nd filter.
most of the time, you will need something such as 10 stops to create something that flowy though. then again, if you're say, in the shade, that'd be another story.
my point is that just because someone achieved a desirable effect with a particular piece of equipment, doesn't mean that using the same equipment will get you the same effect.
This is quite naive thinking. Photography is much more about light (angle, intensity, colour, etc) than it is about equipment.
yes, the nd 110 is probably a surefire way of getting flowy water effects at any point in time.. at least it is one of the strongest nd filters known to us.
the trouble is attaining efficiency. for example, if you only need a 30 second exposure, and the nd110 pushes it to 16 minutes......
in a best case scenario, we would all have various nd filters in our bags for different lightings.
I agree that there is no 1 ND filter that can suit all situations. Like night86mare mentioned, it's good to have various ND filters and adjust accordingly. Alternatively, you can opt for the fader ND which the stops can be varied accordingly.