And Kath, so nice to see you here and thank you for your advice on the filter, I always learn something new from you. Also good that you made it back from Italy in one piece, so sorry to have missed you.
Sorry I've only just noticed this post in the Forum. Kath has said just about everything I wanted to say, but she said it better.
ND Grads are so interesting; no two ever seem to be the same, and each make seems to have a different caste to them (Cokin seems to go a bit purple etc). My 0.6 Cokin and my 0.6 Lee require completely different light values. Out of interest, what type are you using? There is always going to be a variance in stops, between brands, between cameras and of course depending on what type of metering you use(spot, centreweight, average etc). So half the fun is muddling about with them.
I can't say that there is one successful recipe for using them. All I can tell you is what I do:
1. Like Kath, I always use Manual mode;
2. Will usually use Spot Metering;
3. Sturdy tripod;
4. Mirror lock;
5. Will usually compose the scene, meter the shot and then put on the grad filter (although lately I've been getting lazy and composing/metering with the filter already on
6. Also like Kath, I'd never use an aperture of f/5.6 for a landscape/seascape and will always use between f/11 and f/20 - the sharpness of my lens tends to fall off after about f/18;
7. I use Lee hard edged filters (0.3, 0.6 and 0.9), but am coming round to the idea that I should have bought the soft-edged versions.
I've spoken with André (Inasiajones) about the use of filters. He doesn't use them at all and so will meter for the foreground, take a shot, then meter for the sky, take a shot and merge the two in Photoshop. This is beyond my skills, but is certainly helpful when there are things which break the line of the grad filter, such as cliffs, lighthouses etc.
I've just recently bought a B+W Big Stopper something like 15-30 stops so you can use it during the day. This definitely requires composing the scene, metering and then putting on the filter. I'm still playing around with it but having alot of fun.
So I wish you lots of fun and lots of interesting photos.