There's an easy way around the filter-factor compensation: you set the uncompensated exposure on the camera, then every click (except for 1/250-1/400... unlikely when using Velvia!) of the shutter speed wheel is one stop. If you go past 8s then the maths isn't much harder (1/4 minute, 1/2 minute, 1 minute, etc) except that reciprocity gets involved for most films so you use Acros or Provia for the really long shots and mostly avoid that problem.
Originally Posted by Helinophoto
The other thing is that when using an ND grad, I find it easiest to do the exposure calculations on part of the scene that's under the clear part of the filter. Say you have a reading for foreground and sky, the latter being 5 stops brighter. You whack on the 3-stop grad and the difference is now 2 stops so you decide to put the foreground at -1 and the sky+ND at +1. Meter the foreground with an exposure compensation of -1 (i.e. go 1 stop faster than metered) and dial in that exposure. Then if you have a CPL, put two more clicks on the speed dial and close down about 1/3 stop in aperture (for a total of 1.7-stop compensation on the CPL).