I have been trying to learn how to use this lens. ... (
WIth all due respect, it ain't the lens, it's your technique. The lens has nothing to do with getting milky water. Your posted image was too fast, at 1/320th
If you are going to use an 8X ND filter, then download the Lee Big Stopper app from the app store of whatever smartphone you have. The app works with Lee's 6x, 10x ND 15X filters but your 8X will fit just fine with the 6x settings. If you don't have a smartphone, you can download the Big Stopper exposure chart on the web. Oh, you'll need a remote release if you want really long exposures, and a tripod.
Frame your shot, focus and for starters, I put the camera into program mode, get the exposure settings, transfer them into manual mode and take a few test shots to get the flowing water right, but not in a "milky" way. Lets say, for discussions sake, you are at Sunny 16 settings, 100th sec, F/16, ISO 100. The Lee app, for 100th sec, gives an exposure time of .06 sec, which you can do easily on camera without a release. Set your camera for 2 sec delay, click the shutter and go. It is also a good idea to cover your viewfinder with something to prevent light leaking into your image, which will produce a red cast. I've ruined several shots forgetting to do this.
Without the filter . try a 1/4. 1/6 or 1/8th of a sec. With the 8x filter, @ 1/320th, an exposure of 1/4 sec would produce the results you are looking for.
Look at my profiles for recent photographs from Yosemite for examples.
Hope this clarifies things.