I don't have any experience taking long exposures in daylight but fortunately photography exposure is very simple mathematics. i.e. You own a 7D so DLA begins to effect image quality at aperture values greater than f6.3. Let's for simplicity sake pick f8.0. DLA should not be too bad there. This will most likely yield the greatest DOF for the sharpet image. Use DOF master to determine the correct hyperfocal distance to maximize the depth of field in the image your trying to make. I believe, (please someone jump in and correct me if I'm wrong) that ISO 100 will yield the greatest dynamic range for your camera. ISO 50, if available will help get your shutter speed down but you'll give up some dynamic range do to in-camera Raw numerical manipulation. OK, so, you've got ISO 100 @ f8.0 and on a bright sunny day that yields a shutter speed of somewhere around 1/250. ND filters simply limit the light in values ofstops. Add a1 stop ND filter and get 1/125, 2 = 1/60, 3= 1/30, 4 = 1/15, 5 = 1/8, 6 = 1/4, 7 = 1/2, 8 = 1, 9 = 2, 10 = 4 sec. Is 4 secs long enough? ISO 50 will give you 8 secs, add some more aperture (post sharpen) and at f11 you will get 16 secs, at f16 32 secs......


So, a 10 stop ND filter is not unreasonable. Slower is harder, faster is easier.


Hope this helps..