I used a version of Gidde's Ink Bleed technique on a few of my works now. Duplicate the layer, blur it a few px (5 or less) then set the opacity to somewhere around 50-70%. That makes the ink look like it's gone to the "right place" but "bled" marginally into the paper around it like a good liquid ink would, especially since metal nibs can (and do) lightly mark the paper when they are very fine.
Check out my Big WIP Project Neoseilthir in the Regional/World Mapping section.