I agree with you computations. I use ND filters of a variety of strengths. I will sometimes use my free “ND Timer” app to quickly compute exposures when using the filters. The app then initiates a countdown timer for the exposure.The following is excerpted from the buying guide Nick provided us and I don't understand it: "[if] a 6-stop ND puts you in the 2-4 minute range, a 10-stop ND would push the exposure past 8 minutes."
Let's assume the 6-stop ND exposure lasts only 2 minutes. A 7-stop ND would add another 2 minutes beyond the 6-stop ND, totaling 4 minutes. An 8-stop ND would add another 4 minutes, totaling 8 minutes. A 9-stop ND would add another 8 minutes totaling 16 minutes and a 10-stop ND would add yet another 16 minutes, totaling 32 minutes. That's not remotely close to the 8 minutes explained in the buying guide.
There has got to be something I'm missing. What is it?