I think you'll find if you make a "normal" exposure, then do the math, you'll get better results than trying to meter through the ND filters.
for example 1/250 @ ISO 100 and f5.6... then add an ND8 or 3 stop ND filter will give you 1/30 @ ISO 100 and f5.6 - 1/30 is slow enough to slick a waterfall.
If you're looking for streaking clouds like those below you need a much longer exposure, but using the same original exposure, you'd maybe stack and ND8 and an ND400 for 12 stops of slow... you'd end up with an exposure like 8" @ ISO 100 and f5.6, and then maybe stopping it down to f11 would give you 30" or, stopping it down to f16 to get a 1' exposure.