Thanks for the information. It is another way of achieving the same results that makes sense to me so I will have a little play around - I do have live view...
Its all trial and error at the moment! More error than anything!
I had some other tips from another site that may also shed light on the matter so I'll post it below for general consumption until one of the obviously talented photographers on Trek Earth decides to chip in and enlighten us...
One guy said:
"ND Grads still allow for normal exposure for at least half of the scene, and the camera meters the entire scene (in matrix mode) so that half that is not covered by the ND Grad is still giving normal exposure and the camera is averaging between the two.
Also, exposure settings are not fixed. If 1/30s gives you a better image than 1/15 use it, just as long as you realize what you see in the small lcd screen at the back of your camera is not always the best representation of what you see on a larger computer screen. Same for ND filters, they are not exact either, they are created within tolerances. Therefore a 2stop ND filter could actually be 1.8 or 2.2 stop instead."
This makes sense to some extent but how do I trust my calculations when using an ND filter for say three stops if they are not exact... ultimately it all becomes guess work and I can't really trust the accepted "theory" of how to use these things... that's something they don't tell u in the instructions or the "how to" section!
I also realised that another complicating factor may have been the fact that I had Active D lighting set to Auto - this is a feature on Nikon cameras (I use a D7000) - may have lightened the darkened areas when I stopped it down further... I'll try it on a normal setting...
I'll just keep going until I get there!
Thanks for taking the time to reply.