Thanks for the comments guys.
@ atrembla - for the maths...
My camera has a 'base' ISO of 200, but I can go lower to 100 if required. When using a 10 stop ND filter it can be difficult to see the composition if it is attached to the lens (sometimes Live View compensates). Therefore I compose the shot I want without the filter and make a mental note of the exposure I need for the shot.
So for example, if I compose with no filters on the lens at F8 and ISO 200 and the shot requires a 1/400th second shutter for a good exposure, then that's my base.
Then, if I add a 3 stop ND filter I can still see through the viewfinder (as it's not that dark). I know that the shutter speed will slow by 3 stops (or double three times over) - 1 stop reduces to 1/200th, 2 stops reduces to 1/100th, 3 stops reduces to 1/50th.
Then, if I add the 10 stop ND filter I cannot see through the viewfinder, so I have to calculate the additional 10 stops manually. 1/25th, 1/13th, 1/6th, 1/3rd, 1/1.6", 1", 3", 5", 10", 20".
So now, for the same exposure as the original base shot (F8 and ISO 200) by adding the filters I can slow the shutter from 1/400th second to 20 seconds.
Now, if I want to slow things further I can do several things:
1 - add another filter (stacking filters can compromise the image quality so I prefer not to)
2 - change the aperture of the lens (stopping down too much can again compromise the image quality)
3 - change the ISO
I prefer not to stop down beyond F18 if necessary. Here - remember my base shot is at F8, so if I reduce the aperture in 1/3 stop increments I can go from F8 through F9, F10, F11, F13 and F14 to F16. This reduces the light entering the lens by 2 further stops, so therefore I have to slow the shutter a further 2 stops to compensate.
Therefore F8 and ISO 200 with filters at 20 seconds moves to F16 and ISO 200 with filters at 80 seconds.
Now I have one other option to slow things down again, which is reducing the ISO. Reducing from ISO 200 to ISO 100 gains me another stop, meaning I can reduce the shutter speed further.
I can then go from F16 and ISO 200 with filters at 80 seconds to F16 and ISO 100 with filters at 160 seconds.
Clouds are a rarity in Dubai for 10 months of the year, so when there are some it helps with skies. Generally they are wispy and high so I wanted to blur them and induce a bit of movement, which I why I had to go to extreme lengths to get the effect I wanted! I also like to use the 10 stop for watery reflections where the long shutter smoothes the water and gives a cleaner reflection.
The more you stack filters, the more likely that image quality can be lost. Also, when using a really dark filter there is a colour cast as well. That's why I take a base shot without filters to match the original colour as accurately as possible to the filtered shot in Photoshop.
It takes a bit of practise but gives you the opportunity to add interesting effects into shots that have been taken many time before by many people (as in this Burj Khalifa shot) - it's not something you'd use all the time but I find it useful and if I'm out with a tripod then the filters are always in my bag along with a crib sheet below which I set up to help with the maths!