I've not used the LightCraft variable ND (I made my own*), but using a variable ND is similar but different to using a standard ND filter. I tend to choose my aperture, shutter speed and ISO and then adjust the strength of the VND filter when metering. It helps if you're using a fast lens and a body that can meter through a strong ND filter.
* if you already have a circular polariser, buy a linear polariser and put it in front of the circular polariser. Bob's your uncle and you now have a variable neutral density filter. I've been told that theoretically you can get the same effect with two circular polarisers, but a linear is cheaper than a second circular, and in my experience you get more problems/artifacts with two circular polarisers than you do with one circular and one linear.