Hi folks. Which level of ND filter would you recommend for someone who wants to experiment with (a) blurring water, clouds, etc., David Fokos style and (b) “erasing” people from busy tourist attractions?

I use B+W filters and their midrange ND1.8 has a 6-stop exposure factor. I looked up the reciprocity information for my film (T-Max 100) and assuming an initial meter reading of 1 second, applying a factor of 6 would (I believe) mean 64 seconds before reciprocity correction and 207 seconds after reciprocity correction. Based on your experience, is 3.5 minutes in the ballpark for accomplishing what I want to do?

The strongest B+W filter I found is ND6 (20 stops). What would one use a 20-stop filter for?

Thanks!