My understanding is that until you get up to the higher grades exposure stays constant. This assumes you first figured out exposure with the #2 filter in place. Which paper are you using? I know the Agfa datasheet claims that up to grade 4 it's constant. 4 and above it looks like you lose a stop. From looking at it I'm guessing in reality what they've done is put some ND into the lower filters so they print the same across the range. Grade 2 without a filter is much faster then grade 2 with a filter.