since the Sullivan comes with a 5/8 bridge you probably shouldn't go any shorter. I also don't like sanding bridges since I haven't figured out how to put the sawdust back.
From Stelling Banjos......
Step #4 - Checking string height.
Normal string height, or action, is about 1/8" above the 12th fret and 9/64" above the 22nd fret as measured from the top of each fret to the center of the strings. To modify string height entails manipulation of the rim rods which are the two rods traversing the center of the rim. These rods secure the neck to the rim and must always be snug at the neck end prior to any adjustment at the opposite end where the 1/2" nuts are. Two holes in the middle of each rod facilitate turning the rods onto or off of the neck hanger bolts, or enable you to prevent the rods from turning when tightening or loosening the adjusting nuts. To lower the action, loosen the inner nut on the lower rod and tighten the outer nut. To raise the action, loosen the outer nut and tighten the inner nut.