if you go with some of those Hi-tech filters, let me know how are you satisfied. My experience with Cokin-Z who claimed to use nano coatings were really bad…
To serve the purpose of reverse grade using method I described, you can’t use soft grades, as they are loosing density toward area of overlaping. I am not sure I xplained it well, so let me try again:
You take 1 0.9 (3 stops) and 1 0.6 (2 stops). Hard grades should have similar dnsity in its full coverage. Rotate (180* rotation) one of the filters and put it in the second slot of the holder (0.6) than put another filter from above in the first slot. When they start to overlap they will create 1.5 (5 stops) in that area. What we have now is -2 EV bellow, -3 EV on the sky and -5. EV in the overlap area which we can move above or bellow to match the horizon. Basically we are getting -3 EV compesention toward foreground and -2 EV toward sky, and 5EV on the brightest horizon, which in my experience works best.