I'm late to see this, so you've probably figured it out by now, but I thought I would add my twopenn'orth.
I've been using ND grads for a while now and find that I get the best results by using manual exposure mode and spot metering.
Manual exposure because you control the aperture and the speed. With aperture priority the speed will automatically change as soon as you add the grad.
Spot metering because you can metre on the foreground choosing something you think is 18% grey....grass usually. If no grass, the palm of your hand is said to be roughly 18% grey. Only add the grad after you fix the settings.
Using the spot metre also lets you check out the reading for the sky and figure out which grad you're likely to need, although it still requires a bit of guesswork. When using matrix metering the meter takes an average of the entire scene including the sky, but to use grads successfully you're accepting that the sky and the ground require different exposures.
Hope this helps.
Last edited by feather; 10-12-2012 at 12:02 PM.
Reason: missed word