With unusual luck, all three of my enlarging lenses take the same size filter, so I bought a regular 2-stop camera type filter. I work mostly with 6x6cm negatives and usually do some small test prints about 5x5 inches with the filter on. Then to do 10x10 or so, I crank up the head and take the filter off -- puts me right in the ballpark for exposure (not exact, but close).