K-5 and Waterfalls

Hmm..
K5 should have ISO 80 available.

Try to check that highlight protection isn't on.  I think this will limit you to ISO 200 and it must affect RAW file format (with respect to limiting ISO choice but not any beneficial effect) as well.

For JPG, highlight protection can be a good thing.  For RAW, it isn't really beneficial if you are careful with exposure.  You take care to not clip highlights and work on the file in PP.  You are your own highlight protection.

Thing is, if you do RAW+JPG with highlight protection off, your RAW file will benefit from lower ISO and careful exposure but your JPG will be left out in the cold.  Not always, maybe, but in high contrast scenes like your waterfall, your JPG will suffer for not having highlight protection on.  Not a reason to not try RAW at lower ISO but just a thing to think of.

With ISO 100 available to you, ISO only becomes available if you turn on 'extended ISO range' in custom menu.  The K-5 ISO 80 is actually beneficial.  It is not a camera+ISO trick to simulate exposure settings at ISO 80.  DR is a bit greater in ISO 80 than in ISO 100.  A little  :^)

As David did, I started with a lower power ND.
Mine was a 3-stop rather than his 4-stop.
I now have a 10-stop, myself.  It is more flexible for daylight use.  I don't always like the very smooth look and sometimes like a bit of texture in water.  The 3-4 stop ND can be good for this without stressing small aperture too much or for evening pictures.  3-4 stop very hard to get silk water in sun  :^)

But even a 10-stop wouldn't be too much for your first picture.  Dropping ISO to 80 but also opening aperture a couple stops and then adding several stops longer shutter for the darker filter and you should still be within the 30 second metering limit of your camera.  So no timing exposure on the cell phone.  Really, the 10 stop might be good to further protect the highlights in the upper left and for other scenes in more direct sun.

I first thought 10-stop or close to it might be too dark.
If interested, take some pictures you like and work out equivalent exposures with the 10-stop in the mix.  See if it is darker than you like.  There is a convenience hurdle beyond 30 seconds shutter.  I use cell phone timer for that...